My son is turning 11 years old in 6 days and I don't know what to do with him on his birthday. He loves Harry Potter and he also loves DC superheroes. He loves Harry Potter more than superheroes, though.
Rent a Harry Potter movie Have some games and party food. He will love it.
iF you are just looking for something to do, I would take him to a movie and eat.
If you are looking for party ideas, for how many guests? Kids or adults? I would do a movie and cookout and have a cake, or pizza and a cake. You could decorate the cake with either movie theme.
25 years from now I'm betting your son would love the ongoing tradition of .... Why not start something the two of you can do every year on his birthday. And won't you both love looking back as the years go by. Could be something entirely separate from a party. A movie (he picks) and a meal (before or after) which you cook and he chooses? Could be the two of you cook together? Why not ask what he thinks? Wouldn't we all have loved to spend more one-on-one time with our folks.
it would be fun if you could have some sort of harry potter costume party, though it may be too late for that already
aside from that, have a harry potter movie marathon would be great. Not very active, but very low maintenance.