If your boyfriend made a commitment to spend that time with his son, then he needs to honor that commitment. It's natural to feel disappointment over this as you want him to be with you on your birthday. However, since I'm not sure when your birthday is, now that you know ahead of time that he won't be with you, plan something for the two of you to celebrate.
At least he told you in advance what was going on. Be thankful for that.
