Welcome, Catherine!
Graham's obsession (as we jokingly call it) with numbers, letters...now progressing to how words (all of them!) are spelled, and the concept of telling time has been completely self directed. We answer his question, provide him with computer time (limited), a never ending supply of books, etc, but never really sit down and drill him per se.
Our DS6 has always been a kid with "obsessions," which is what we called them, too. LOL! As long as they're child-directed--as it sounds like they are for you, too--you're not hothousing. You're just following his needs where they take you.
I have no idea what to do. I am at a loss. As each day passes and the mental jumps he makes get larger and larger, I get more scared that I'm really going to screw this up. Either by downplaying it too much for fear of being labeled as one of "those" parents, or focusing too much on it and not giving him the normalcy I want him to have.
What you're doing is exactly right. It is scary, but you're doing fine! Don't overdo the pressure (and I can say that because I have felt it!). He's a kid and you're a mom. Your job is to be sure his needs are covered, regardless of what those needs are, just like any other mom does with her kids. Is it a challenge? Sure. But you are already demonstrating that you're up to the challenge!
I joke that I KNOW I'm going to screw up my kids--I'm a mom and that's our job! LOL! But I try to remain consistent in HOW I screw them up so that they save money on therapy later. It's a lot easier to fix one thing that's broken than lots of things.
But seriously, all you can do is your best. If you're trying and you're being self-reflective about it, you're doing all you can do, and that's all a kid really needs. Love, limits, and parents who are trying their best.
I will say that I came to the conclusion fairly early that I had two choices: be seen as one of "those" parents, or have my child become one of "those" kids. I opted to carry that particular burden. I think it pays to make the decision early, because worrying about how you're perceived and effective parenting don't go very well together in my experience. FWIW...
Anyway, welcome!