I can't say that I've observed this in either one of my kids. DS has various "fascinations" and tends to ramble on. So he might be talking about a certain video game and then a couple minutes later he will be talking about space stations. Then later on, it might be the video game again. But he doesn't seem "obsessed" in the way that you're talking. DD will get wrapped up in something and seem to hyperfocus while she's doing it, but then not think about it at all when she's not doing it. I think what you're describing might be normal if it's just a short phase, and he is interested in other things as well...for instance he is currently learning the numbers and highly interested, and the topic will come up several times per day. But then he will quickly move onto a different interest. However when you say he will write numbers all day and it seems to calm him down...that seems like a huge red flag. Have you looked into OCD?
