I have always had trouble shopping for greeting cards for that very reason! They've always been too mushy for my taste.
Some stores (Hallmark, for one) do sell "simply stated" cards, which are the ones I tend to go for. Usually they say something simple like, "Have a great Mother's Day," or something like that instead of the usual long-winded ones.
He's struggling with what to write. He doesn't just want to write "Happy Mother's Day", but he also doesn't want to lead her into a dramatic round of he said/she said by getting into why he hasn't been speaking to her.
I don't see anything wrong with a simple, "Happy Mother's Day," but if he wants to write more, he could say something simple like, "Thinking of you. Have a great day."