Right now, my mum is developing dementia (we're waiting on tests to help with a definitive diagnosis). Her memory is very bad, and getting worse. She can forget what she said in one sentence by the time she is part way through the next. If it wasn't terrible it would be amazing - like she fully 100% has no idea what she said and sometimes will even argue that she didn't say that thing. So I think it's completely within the realms of possibility that he doesn't remember.
I also think that he doesn't care enough about any of this to even register the information as worth remembering in the first place. He's a shitty cunt regardless of the objective assessment of his mental state.