SO's little girl...

My SO's little girl is turning 3 tomorrow, but the problem is that his ex is keeping the child from him. It's just a horrible, horrible custody situation. He hasn't even seen her in nearly a year.  Well, tomorrow will be the first bday he can't celebrate with her and he wants to essentially "skip" the day. I worry he'll regret that decision and I want to somehow acknowledge this special day for him without aggravating raw feelings. Not sure that's possible though. Advice? I don't want him to skip a day of significance...