Please accept my condolences. Just because your relationship was as an ex-spouse, it doesn't change that you grieve and feel a sense of loss. That is probably exactly why you
feel left out to have not been mentioned in the obituary.
Many times the obituary is written with the guidance of the funeral director and this may have been what happened when your son was at the funeral home to make the arrangements. It is uncommon to include an ex-spouse in an obituary, which is what may have been the thought of the funeral director. That doesn't mean it should have been the case here but maybe your son simply followed his/her suggestions, so maybe you can give him the benefit of the doubt.
If you feel you need to share your feelings with your son, you may try saying something like, "I know funeral directors usually help people write obituaries and they usually don't mention former spouses, but I felt kind of left out by not being mentioned. I know you understand." Smile and leave it at that to avoid making him feel badly about something that can't be undone. You're very fortunate; it sounds like you had not only a good relationship with your ex-husband, but a continuing one with your son.