Sorry to hear that you weren't recognized on this special day, but since I do not know your DH, I won't say he is bad for not getting anything. If he doesn't get his own mother anything, it sounds as if he was raised not to honor the day....there are a lot of people who consider it a "Hallmark" holiday, meaning one thought up by the card, flower, and candy companies to boost sales.
I would talk with him calmly about what the day means to you and why you feel it is important to honor both Mother and Father days in your household. Hopefully, once he realizes that it is important to you, he will make an effort to honor it from now on. Explain what you explained here, that you aren't looking for huge expensive gifts (although they will be welcomed

), but just the recognition that you are the mother of his children.
Oh and as for the "your not my mother", well remind him that there are lots of people who honor the special people in their lives on Mother and Father days. I always gave a card to the lady who lived next door to my family when I was growing up. She watched me everyday after school and I always considered her a second mother. When I was confirmed, I even asked her to be my godmother for confirmation. She wasn't my mom, but she was very special to me.
I also send my sister a Mother's Day card every year. She isn't my mom, but she is the mother to my two adorable and well loved nieces. I always recognize that because it shows her that I cherish both her and my nieces.
Same for Father's day. My stepdad didn't raise me....mom and he started dating when I was in HS. But we are very close so I always get him a Father's day card....and I know it mean a lot to him...I will always remember the tears in his eyes when I gave him his first card (before they even got married).
There are other examples that people have written in their response that can show him that it is just a recognition of love for being someone special in someones life.