My XH is exactly the same with my DCs, on Christmas Day couldn't speak to them would only text. Never asked them what they'd got for Christmas just a whole list of the things he'd been given.
No phone calls on birthdays either - although they did get a card and a present. Christmas present was 40 euros each - so a lot of thought here. They're only 12 and 10 too.
My oldest DC has now declared he doesn't really want to see dad again - but even when XH phones he never asks to speak to DS1 or even asks how he is (he only Skypes about once a month).
Their loss I think, they're missing out on wonderful children. I have to say that after three years my DCs just seem to regard their dad as someone who rocks up now and again and as somebody who makes fantastic promises to them but they never fail to materialise.
I wish your son a very happy birthday for tomorrow. I expect by now he won't even notice his dad hasn't bothered he'll just be happy to be with you and the fuss you make of him.
It's us as the mum's who feel the saddest I think.