My son loves Castle Crashers...you are on a quest to save the kidnapped damsels...it has potions, puzzles, aliens, wizards, crazy sock puppets, a sword arsenal that makes me laugh, animal side kicks you collect to help you along the way...and it is just gory enough in a very cartoon like manner to appeal to a boy but not realistic at all. My son has played it through three or four times.
He also likes Costume Quest...it is Halloween night and you are trying to save your sister who was kidnapped by evil trolls who want all the candy for themselves. You collect things, find things, fight trolls, solve puzzles and save your sister. Once again very animated and has enough violence to appeal to boys but not be too graphic.
Also he likes Mini Ninjas, the Lego series games (we have not done Harry Potter).