I agree with Merry. If you were talking to him all day and didn't give him a gentle reminder, he quite possibly felt trapped. I also wonder how you mentioned it.
I've had my husband forget my birthday. He even forgot my 30th last year. I got my pressie - a tattoo - a month early, and on the day he didn't remember immediately. I think I let him go an hour before I asked "have you forgotten something?". It took him a moment to remember but he was very apologetic and had time to make up for it. Chances are the fact that you waited until the end of the day made him wonder 'well what am I supposed to do about it now?' Did he even say 'oh, happy birthday.'?
'People are never perfect but love can be. People waste time looking for the perfect lover rather than creating the perfect love' - Princess Leigh-Cheri from Still Life With Woodpecker.