Five-year-olds usually like, and benefit from, something that will keep them interested and busy. A building set or a play set that would let him re-create some real-life scenarios might keep him busy. Since he's a boy and five, he's probably beyond something like a play set that would let him imagine a family setting and people doing things (in other words, a doll house kind of thing, which three-year-olds of both sexes tend to like). A five-year-old boy is more likely to be interested in something like a play set that that would let the "people" in it do things involving, maybe, working in construction or on cars - that type of thing.
As far as building sets goes, Legos (or something similar) usually keep most kids busy. Something like one of PlayDoh sets (or that type of thing) can keep a child that age busy.
If there's a character (maybe on TV) that's popular among kids his age, anything associated with that character would probably make him happy. If the character shows up on whatever the item is, he'll probably be happy with hit.
Puzzles for kids 6 years old might keep him busy. Something like a set that would let him make something (but that's also easy for a five-year-old to use) might be good.
Things that let a five-year-old be active are also good. Whatever you get should be something that doesn't require another kid to play with, because it's better if he can play with it when no other kids are around too.
If he's got an interest in one particular thing (like baseball, for example), you may be able to find a book with lots of pictures in it (even if the pictures have simple captions he's not ready to read) of baseball-related things. Any "gift type" (more special than, say, Golden books) book with pictures might be good. You'd just have to look at it to see if it were something he could browse on his own. Sets of books are good. Games for kids his age can be good. An inexpensive (even toy) keyboard that would let him try to make music might be good. A simple magic set could be fun for him. Art sets. Clay. An action figure that comes with stuff for the action figure to "do". All things a five-year-old may like.
Toys/activities marked, "for ages 4 to 6" are generally good, so watch for that on the package.