NO!!!!!! You did not overreact :) You under reacting in my opinion!!!!
It doesn't matter that your child is small, he can't buy or make a gift on his own without help from someone (ie significant other). And, if he doesn't start doing special stuff for his mom now, when he is old enough to do it on his own he won't because he won't understand or see the importance. I'm sorry that your husband's mother passed away, but that doesn't give him the right to "forget" mother's day for you,
I have two small children and they each give me a card and gfit (via daddy buying it of course) for mother's day, birthday, christmas, any other holiday that gifts are done. But then again, we (husband, his family, me, my family) are all like you and holidays and celebrating family members is always a HUGE deal for us :)
I don't know what to tell you or offer any advice on how to make things better in the future except maybe seek out a professional on the matter (some churches offer cosultation/counseling/etc and it's usually free).
Good luck :)
~A.