Facebook is a pretty impersonal way to make contact - especially for important occasions like a birthday. Telling him in person or by phone is a more appropriate way to do so anyway.

I would be uncomfortable being a Facebook friend with a young person - relative or no relative. It feels like such an invasion to me. I have seen things I would much rather not see and I have been the one to do the unfriending. Likewise, he may have been uncomfortable with you seeing his college/university lifestyle. Don't take this as a personal slight. It is probably a good thing you aren't connected on Facebook in the first place.

Finally, please don't take social media so seriously. I can't stress enough that it is not a replacement for direct communication. While Facebook is fun, and a nifty way of being in the loop, it should by no means be a primary source of maintaining contact. So, use the phone. Mail a greeting card. Wish that happy birthday in a way more personal.