I'm going to judge this two ways. Firstly, you never said in your post that you have mentioned it since the first date. That could explain this, because a first date is a really nervous experience. He might have been so nervous in fact that he simply just forgot it. However if you have told him, he doesn't really have an excuse...

Take him to her grave or marker, that will give him what he wants

dont feel to bad OPVots hard to remember things that happened on firt dates sometimes because they are nervous

the dead mom trick.... classic!

I'm honestly surprised that anyone thinks that this is something you can just forget. He didn't forget a small, insignificant detail like your favourite brand of salt. He forgot a piece of information that is extremely important and that has significantly influenced your life. Even if he forgot everything else about that date, he should not have forgotten this. I'm sorry about your mother, though. May she rest in peace.

You should dump him

I say dump him lol

Epic fail on the attention span.

Hey at least you know he doesn't listen to you

Even if u hadn't told him this, why on Earth would he think it's mandatory to meet your mother after only 4 dates?!