I'm so sorry you were ignored on Mother's Day. That really stinks.
I feel your pain because I, too, am ignored almost every Mother's Day. Even though my kids are still little and I'm in the daily grind of being a SAHM, my husband does ZERO for me (or his mom) by way of acknowledgment or appreciation. I have to do everything. And I'm sorry, I just don't buy the "it's not in the male genes" b.s. That's nothing more than a cop-out for guys to be lazy and selfish. I don't know how women tolerate it.
I have to have THE SAME conversation with my clueless husband every year and tell him that I am hurt to be ignored on the ONE DAY that I am supposed to be appreciated. Lord knows I don't get it all year. I don't think one day is too much to ask. I tell you all this so you know that you are so not alone!
Your son is an adult. All you can do is let him know how much it hurt your feelings to be ignored. Don't guilt him, just be straightforward. Let him know that on Mother's Day, you expect some kind of acknowledgment, be it a card, a phone call, a gift, a visit, whatever. Something - anything - to let you know that you matter to him.
Maybe times have changed. When I was growing up, we went wild on Mother's Day. My mother was not allowed to lift a finger all day. We all waited on her hand and foot, cleaned the house, cooked her meals, rubbed her feet, etc. Basically all the stuff that SHE did the other 364 days of the year! And she deserved it. Now as an adult, I wouldn't DREAM of letting Mother's Day pass without a card AND a gift AND a phone call to my awesome mom. Anything less would seem like a slap in the face. But maybe that's just me. :-/