If you don't mind me asking, what's his reason?
girl, you boyfriend doesn't trust you, or is hiding something from you
its not stupid at all. If he adds you then he risks you finding about his other gf's. Duh
Definitely hiding something
Eggtually, if the reason for not adding you is that he's afraid that facebook misunderstandings and drama might ruin your relationship, then I can understand. Maybe he cherishes you so much that he doesn't want to take that risk? Or maybe, instead of FB contact, he prefers you communicating with him through phone or face-to-face. So, if he would ask you how your day had been, you would be telling him something new. Else he could have read it all on FB.
On the other hand, he might be ashamed to show all his FB friends that you're his gf... Hmmm. You should really talk to him about it.