There are many appropriate themes for a birthday party for a boy. Some are appropriate for younger children while others are appropriate for older children. As children get older some ideas might be embarrassing to have known to their friends even if the theme is one the child enjoys. And some themes elude the younger children.
One thing you can do is mix things up. If there are to be a range of children attending the party, get some supplies for one theme, other supplies for another theme, and so on. There is no limit. And if it is to be a party with both boys and girls attending some things appropriate for the girls might even be a good idea.