Leave him be! I have three boys and all differed widely in their interest in doing sedentary, creative things. My middle boy - who so far looks to be the one with the highest academic aptitude - would NOT color, draw, paint or write voluntarily until halfway through 1st grade. I really thought he'd need OT. While my oldest loved Legos and Tinker Toys and elaborate race tracks, and my youngest loves puzzles and things like Magnetix, he wasn't interested in any of those things (and still isn't). Then halfway through 1st grade, he started drawing all the time. He only draws things like hockey players, but it's all day, every day. Turns out he's a bit of perfectionist and didn't enjoy drawing until he could make his hands create what his brain pictured. He's supposed to be getting ready for school now and I just had to take away his clipboard and pen because he's been drawing for a half hour. The only other "imaginative" play that my middle boy engages in his playing with is set of hockey guys. Hockey is his thing, and we let him indulge in that.
Let him take the lead - if superheroes are his thing, get some of the Imaginext things (like the Batcave) so that he has more props for creative play that you can engage in with him. Get the superhero Legos, the Star Wars memory game, etc. Meet him where he is. Some other alternatives to traditional crafts are Moon Sand and Play-Doh. My boys all like that more than traditional crafts - they're very tactile and you can sit at the table with him. Don't be surprised if all he makes are super heroes and weapons though.
One of the things I've learned from my sons is that it's important to value what they value and validate their interests. My oldest went through various phases of obsession - oversize construction vehicles, sharks, dinosaurs, disasters like The Titanic, baseball, hockey etc. - and I had to develop or feign interest in those things. He's now 13 and even now will occasionally say "you really weren't into dinosaurs? I thought you knew everything about them! We had all of those books and movies..." so it can make a big impression on them if you engage in their interests, no matter how boring, unimaginative and repetitive they are. To this day my oldest really thinks I can tell the difference between when he takes an "Ovechkin" shot vs. a "Chara" shot in hockey :-) As mentioned, my middle son is obsessed with hockey. My youngest, thankfully, it a bit less obsessive and has a wider variety of interests.
Don't feel bad - sounds like he has lots of enrichment and social time already in his day. If he spends his time from 3 to bedtime in superhero land, that's OK.