To be blunt about it: it's possible that you may not be able to.
Those sorts of interests are all-consuming. And it often feels not only difficult, but utterly pointless to interact with others when there are no shared interests. The constant rambling about an interest can be an attempt to create that shared link, and it can take awhile before one gives up.
Eventually, he may stop. But if so, he may simply withdraw and entirely avoid others.
While I myself can appear talkative and such on the forum here, even I have this trait. In person, I dont interact with, well... pretty much anyone. Family? Only the absolute closest will ever manage to get me into a conversation, and even then, only rarely. Others beyond immediate family? Nothing. Nothing at all. I simply dont experience any interest in interacting with them as I cannot see any purpose to it. They're not interested in what I'm into, and I couldnt give less of a fart about what they're into (which seems to be exactly 3 things for pretty much all of them: sports, politics, and pointless gossip).
With no shared connection of that type, conversation simply doesnt happen. What in the world would I talk about with them? They'll sometimes attempt "small talk" of course, but I dont understand the point of that silly ritual, so I'll just try to politely deflect it and then wander off.
There CAN be exceptions to this rule, of course. Even now there are a couple of exceptions beyond immediate family. But they are very rare.
This is all just based on my own experience with this, mind you. The exact nature of it may not be quite the same with your nephew.
It's certainly worth a try at least, I guess, just dont get your hopes up too much, is what I'm saying.