Birthdays go two ways. In the past I've called my mom on my birthday, sent her a card, and once, in a fit of gratitude, sent her flowers. (Tho I admit, I haven't done any of that recently). It's an acknowlegement of what she was going thru X years ago, and if she hadn't gone through it, I wouldn't be here.
It is depressing when someone important to you forgets your birthday, tho. SO has a mental block about my birthday (for three plus decades now), and seems to have the idea that it's on the 21st and not the 19th. I know this about him, and will usually remind him, but I confess to doing the PA thing once or twice over the years and just not mentioned it. Talk about an exercise in self-control, going all day without saying, "it's my BIRTHDAY, you big stupid lug".
OP, would it be completely out of the question to just call his mom and say "Your son was bit down because he didn't hear from you, would you give him a call?" It's not graceful, but it's better than seething.