Hi, I think that it is hard to tell exactly why your step son may be doing those things. It's likely a combo of a few factors. I can say that not all bipolar people are messy, inconsiderate or manipulative. Many are the opposite of that, usually. My roommate and cousin are both very neat and clean, but that could be because they have OCD as well.
The uncooperative and belligerent attitude that your step son has could be from his BPD, if he is perceiving that "everyone is against him," he may be acting out in response to that belief. It could cause angry outbursts or refusal to not make a mess in the house. If his medicine isn't working, or he isn't taking enough, he's more likely to feel that way. He could also feel overwhelmed and depressed, and not "want to be bothered" with anything. He could be messy because of tiredness from meds, especially Depakote or lithium, but not enough to be doing it to the degree that you mentioned. I know a few people on those meds and it does not make them slobs, just slow and tired. I also don't think it would make him uncooperative or angry. His behavior could just be due to his personality. I think it is likely due to a lack of proper treatment for his condition.
For example, when my roommate takes his meds as prescribed, he is much less likely to get angry and to act out when things aren't going his way. He is very pleasant, caring, co operative and reasonable. Since he has gone from taking 2 pills a day, to half a pill a day... plus alcohol and marijuana, his attitude has changed a lot. On less meds, he takes things personally, and sometimes believes that people are judging, criticizing or hate him....when they are only making a neutral statement or doing something that has absoloutely nothing to do with him. I am careful when discussing things with him because he can get offended easily. I frequently have to tell him that I am not accusing him of anything, I am just talking about something that I have observed as a problem and need to figure out how to fix it. I often have to remind myself that everyone's perception of the world is very different than mine, and that his reality is especially different because he is not taking enough medicine AND the alcohol makes him angrier. I've seen him off and on both alcohol and marijuana.....and notice that we get along much better when he's only smoking marijuana. On less meds, he also says he is overwhelmed with things and "just can't be bothered" often....with relationships or doing things for himself or others that would normally be seen as necessary or important. He also seems to care less about things and people in general.
It's also possible that your step son could be forgetful of things he's asked to do because the meds could be affecting his memory and cognition. Both my cousin and roommate have memory problems on meds. My cousin is on a few heavy meds and he is so affected by it, that he speaks very slowly and forgets things in mid conversation. It is very difficult for him.
From what you've said, it sounds like your step son isn't seeing his psychiatrist regularly enough. People I know go once a month, unless they are stable, and then its every 3 months. All of them go to psychotherapy once a week. As far as being able to tell if he is taking his meds properly, that's very difficult. The only way to know is to see how much he should be taking, and check the bottle every so often. And that's only if you see him taking it. He could be throwing them out if he knows you are paying attention. My roommate lies about what he should be taking. I see him take meds most every day.....but only a half pill. Because I knew him when he took 2 pills a day, I can tell the difference just by being around him. If he hasn't taken any yet, he's a lot more energetic and says a lot more funny nonsensical stuff than usual. I don't mention his lack of meds often, because I cannot tell him what to do. It is his right to live as he chooses. But, I don't put up with threatening behavior and will tell him when he is becoming too manic. On one occasion, he was threatening me, screaming a lot, slamming things around for half an hour, and just wouldn't calm down....I told him I would call the police if he didn't stop. That shut him down right away and he's never done it again. His exes had called the police on him before, and he knows he could be taken either to jail or the hospital. Also in NY, the police make the violent or threatening person leave the house, if they are not arrested or taken to the hospital. When I notice that my roommate is particularly manic and becoming obnoxious, I always ask if he has taken (what he refers to as) his "fruit loops medicine" yet. He will laugh and then he usually takes it. I've also talked to him after episodes he's had.....the next day after he's calmed down, and before he's had any alcohol. I say that I've noticed that his meds may not be working so well for him anymore and that he may not be aware of it, but he seems to not be in control of things as much as he was before....and he should get it checked out by a dr. I also remind him that he should be careful because he might do something to jeopardize good situations he has in life now...very good job, good relationships w/ kids' mom and kids, etc. He still has not gone to see a psychiatrist in years and says he doesn't want to go. He probably won't go unless his primary care dr makes him go, in order to get his medication refilled. But, I think I help him to be aware of when things are getting out of hand, as he seems to reel himself in for a little while after. Perhaps some of these strategies might help you with your step son.
I have seen that people are only going to get help and follow treatment plans when they want to. They cannot be forced into doing anything...at any age. My roommate has been doing that and has been getting away w/ it for years. He is lucky I guess. My cousin, who is 40 and has worse BPD, has been through several bad episodes since he was a teen. He has been on many different meds and taken himself off of them a few times for different reasons. He's also tried a lot of meds that didn't work. Each time, he has had a major episode and ended up in the hospital. Most of the time he's stayed there 3 or 4 weeks. He is more compliant with treatment now, and is hyper aware of his mental state, because he doesn't want to go to the hospital again. The doctors saw that each episode was worse than the last, and said that he might not be able to come out of the hospital the next time he has one. So, he is basically scared into compliance.
Hopefully it won't come to that with your step son....and hopefully some of this info will give you some ideas on how to handle him. Sorry it was so long! Good luck!