On your birthday the people who love you should make you feel loved and appreciated and try to make you happy. Whether it be spending time with you or gifting you with something they think you will like or find useful. None of which your SO has done. I'd definitely bring it up, if I were you I'd go and do it now. Calmly. Without accusing or anger so that he has no need to get defensive. Hey SO, I always try and make you as happy as I can on your birthday and as much as I can't wait to eat this cake, I'm quite upset that all you've done is got me a cake. I don't expect materialistic things but I guess I thought you would want to spend time with me, or take me somewhere to celebrate today and make it about me a little or at least to make me happy.

Blah blah you get the gist right? Hope all goes well and please leave an update I would love to know his reasonings for this.