C.K.
Cash or check gifts are the best for this. Go in multiples of 18 as it is good luck in Jewish mysticism. I would give $18 or $36. If you write a check, his mom will probably put it away for college, while usually the kid keeps the cash. Either that or a donation is always really nice...not enjoyed by the bar mitzvah boy but really appreciated by the parents. I got a star for mine, and while I thought the donations in my name were stupid at the time, I've grown to appreciate it.