A lot depends on your son, how much you want to spend, and how you want to spend it. Do you want a gift that he can keep eg a watch or a bracelet, or something that he can remember eg a race day.
there are many sites offering a wide range of 'days out' doing things like hot air ballooning, go carting, driving a racing car etc..... do you think he would like something like that? It also depends on how much you want to spend as already mentioned
When my son was 21 we bought him a personalised number plate - just one with his initials so it was the cheaper ones, but he can move it from car to car.