Hi A.!
I don't think it's weird at all! In fact, I think it's awesome that he has such imagination at his age! Trying to take it away from him, even in public, might damage his little ego. I can understand your hubby's embarrassment, but I assure you and him, that as a parent, I love it when kids use their imaginations. Single people may think it's weird, but they don't count...LOL They don't have kids, so to me they have no say so. HA! I saw at Target that they had this batman costum with mask and cape and the little outfit just for 10 dollars! My son's eyes lit up when I showed it to him when I got home! He absolutely loves it! And not to mention, he will be wearing this for Halloween and instead of a costume costing me an arm and a leg, it only cost me a finger...LOL
Let him have fun in public with his costume. I just may encourage my son to do the same thing. Never know, they just may become real heros when they grow up! Encouraging a child to have fun, and be who they are, or use their imagination, no matter what anyone else thinks, is teaching them to be who they want to be without having the influence of others to discourage him. That may be something you can say to your hubby to help him understand that it is ok for your son to be his super hero for the day. :-)