Assuming you're close to him, $50 is appropriate. But if that's a stretch for you, I don't think $20 would be at all inappropriate. It's a special occasion, but at least in my family not one of the big gift-giving occasions.
We are all very close and I gave him and his sister both $100 when they had their communions ( I'am the nieces godfather). The communions seemed like a bigger deal too me, they had parties thrown for them and a lot of invitees, for this close family only are going out to dinner in the middle of the week.