I agree with LH19. It is mother's day, so a gift from your son to his mother is appropriate. You are the conduit to make this happen, and insure it does happen. It should not be romantic at all, and should be "signed" from him.
From a young boy perhaps some flowers or candy if you can't identify another idea.
The only exception might be a store-bought simple card from you that only says something very simple that acknowledges her role as mum. "Happy Mother's Day. (Son's name) is very lucky to have you as a mom"
