Wow, ladies, such great advice here so far.
S., make the day you dread (and maybe even the following one) a day that's very special for you; you can celebrate what a wonderful job you have done with and for your son to date. At one big event in my son's life I realized my husband was clueless as to how I was feeling...so I bought myself a beautiful bouquet of flowers to enjoy. When he asked about the occasion, well, I TOLD him and he was sort of, well, DUH.
I also second the idea of keeping a journal. This is a unique time in your and your son's life and writing your thoughts down will help you deal with the emotion as it all comes out and onto paper. There have been studies that compare how effective journaling is vs. seeing a therapist. It might also be something your son will enjoy reading at a later date...so in a way it could be a gift. I frequently write letters to my son and then put them away for him to have should anything unexpected happen to me; I want him to know how much he's loved.
There are many men out there like you describe; some of my friends NEVER even get a birthday card from their spouses. It's painful for each of them but it seems like it works to make a celebration of your own with friends for these events; in a way, it's sad to think how much joy these men are missing.
Congratulations and good luck as you move forward to this new phase in your life!