Depends on how close you are to the birthday boy and/or his family and what you can afford. How much do you usually spend on birthdays outside the family? That's a good measure. I think of 21st birthday as being special but would only make the gift special if I were giving something that would be appropriate for that birthday and not others. For example, I would take someone out for drinks at a posh place or a place that they want to go to. But cash is just cash and I'd give whatever I'd usually give for any other year and person in that level of friendship.
Unless you're wealthy, I'd give $25 or so if he was just the child of a neighbor I wasn't close to.