Happy belated birthday to you and also happy birthday to your son.
Sure, it's normal to feel anger towards your mlcing spouse when he's not around for special events, etc. I wouldn't allow that anger to fester and raise its ugly head on his birthday. Enjoy the day w/your son and know that he's turned out to be a well rounded and exceptional son.
Just remember, it's his loss and he's the one that has to live w/that memory.
Enjoy your day!
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.