Some information from someone who still eats very picky!
I'm 24 and I still have a very odd diet and refuse to eat certain foods. When some foods are mixed together I can't eat them.. When two different foods mix it can actually taste different and have a different texture. Rather than focusing on getting him to change his eating habit, work to understand what is acceptable texture wise, etc. Learn to separate food for him and such.
My advice, don't expect him to eat multiple fruits or multiple veggies, but focus on finding one he actually likes. Asking him to always try new foods will only provoke more anxiety. That's what I've experienced personally. The more people asked the more likely I was to refuse it, even if it was something I'd try on my own if nobody else was around.
Find a way for the idea to try new food to be his. If you hide food and not tell him he will only become more aware of you attempting to trick him, which will result in him refusing food that he may actually like. Also, remember just because you can't taste the difference in mixed food doesn't mean he can't.
I don't believe you need to work on his tolerance of food. That in my opinion will just create mistrust and anxiety about eating food which can lead to more problems later on in life. My parents eventually found a few items from various food categories, veggies, fruits, bread, etc etc, that I actually like.
Also, expect him to eat the same food obsessively and expect it to change at random!

It happens.. Just as long as it's not unhealthy I don't see it as a problem. I still eat this way. I find something I like and enjoy it regularly for about 1-2 weeks before moving on to something else.