C.M.
There's something magic about the number 18 -- I've always been told that if you give $$$ for a Bar/Bat Mitzvah is should be increments of 18. So $54 would be better than $50. I think "18" in Hebrew translates to something good, like "life".
If you can think of a material gift he would really like, (something related to a hobby of his, or an activity/interest you have in common) that would work, too.
Enjoy!