Buy him a 1TB external HDD with a note saying "This is to back up all your porn, I hope 1 TB is enough"
I very much agree with this ^^
Make sure you don't forget a tin canteen marked "XXX" on the side.
Get a box of condoms in the smallest size possible, and put a rubber band in it for him with a note saying "just in case".
Yes, it's mean. But he's 20.
I was thinking something along these lines.
You know what they say about great minds...