I agree with Beth. Remembering your XW's birthday for you son and taking him to get her a gift is the way it should be. This has more to do with the raising of a child than about your X.
That being said, I see no harm in getting a gift for someone you care about. If you feel like recognizing her birthday then you should. Remember, if you are ever back together (something you appear to be doubting) she will remember the good things you did. I'm sure you don't need to guess whether I got my X a b-day present. :P
In the end, it's not the years in your life that count. It's the life in your years. Abraham Lincoln
It is hard to fail, but it is worse never to have tried to succeed. Theodore Roosevelt
