Is he hearing the words 'wrongly', or is it that he can't get them to 'make sense'.
A sign of an imminent seizure for me is that sentences that people say to me don't make sense to me.
One example is my DS1 asked me the other day (around 4/5 hours pre-seizure, not that I knew that at the time) "I'm waiting for Mr Magic Man". Or that's what I thought I had heard him say. Turns out he had said "I'm waiting for you to get me one".
Now, my ears were just 6 inches from his mouth, he wasn't muttering, and I'm not that deaf...
My brain just can't process language properly when I'm going to have a seizure. I can't speak properly either - I will have word placement issues - "pass me the telly changey thingy" instead of "pass me the remote", and also whole sentences come out as gobbledygook - "iblurgurple" instead of "I'm tired".
Would be worth him keeping a seizure diary and noting down when he notices this issue...
My guess is it's some sort of pre-warning.
(Just a guess, mind you, not clinically trained!!)