A vacation, even if it's a short weekend trip, makes a great 21st birthday present for a son. The gift signifies that your son is an adult and that your relationship can be more of a friendship now.
A birthday vacation will also give you and your son some time to catch up on each other's lives and to get to know one another better. This is especially true if your son is away at college. Small gifts like mugs are also ideal for a son's 21st birthday, since he can fill the mug with coffee, beer or any of his other favorite beverages.