Remember that character from the goofy "Austin Powers" movies: "Mini-Me"? The pint-sized version of the villain? Your husband apparently doesn't want a new individual in the household; he wants a mini-me.
He's so clearly reliving his own childhood through his son -- and that's all the more obvious because the bones of contention here are a child's tastes: Superheroes, movie characters, sports choices. I note that you do not say dad did this with his daughter. Again, son equals mini-me in every way -- right gender, so he should have the "right" interests and tastes too, in dad's eyes.
Is your husband the kind who could possibly be made to see this? Or is he just unable to look at himself objectively? It sounds like you have tried to get him to see the issue here and to ramp back the "do what I would do" attitude and he isn't going for it.
Here's a suggestion: Does your husband have a male peer whom he respects and likes? An old friend; a brother; the karate coach if dad's friendly with him; someone who knows your son as well, and who is, to be frank, more self-aware than your husband is? Some guys (sorry but it does seem to happen with guys) need to hear from another guy that what they're doing is just not right. Your husband is so wrapped up in his mini-me view of your son that he's not hearing you, his wife, when you tell him this isn't working and is hurting your son. So it might mean you need to enlist that male friend to have that talk. It needs to be someone to whom YOU feel close enough that you are comfortable having a serious talk about this and asking him to intervene with your husband. And it needs to be a guy mature enough and discreet enough that he will not squeal on you and say "Your wife wanted me to say...."
Since many guys don't do well with the "let's sit down and have a big, emotional talk" thing, it might work best if Friend is at some event of Son's with your husband -- a karate meet or class would be perfect -- because then the opening is, "Your son is getting so good at karate and he really seems to love it -- it just clicks for him. Hey, I know you were all into having him do soccer because you like it, but isn't it great that he's found his own thing for now?" and so on.
Sorry if it sounds like stereotyping to say that many guys are not great at emotional sit-downs or need to hear certain things from male peers rather than from their wives. But I just find it's true a lot of the time. If your husband is like that, I'd try enlisting some help but it has to be a guy you can trust to handle it. Maybe a guy with a son older than yours, so he can talk about how his own son has established his interests.
I had to smile when I read your post, though. My daughter, 12, has been totally into ballet since she was four and now does it as a major activity; it's her life, outside school. I knew nothing about it when she began, but have learned so much in the past eight years, and have come to appreciate and enjoy it. So your husband might learn to appreciate the things his son does that aren't dad's personal "thing" -- if only dad could open his mind and really watch what son's doing, rather than thinking all the time, "Son should be doing X right now and not this Y stuff." Our kids really can introduce us to new worlds!