Was it a severe hypo? That is, did he go very low or drop suddenly, or even pass out or have a seizure? With a bad hypo, I think it can affect your memory for a day or so, and make you feel rough.
I haven't heard of the speech problems so long after though. Has he checked his blood sugar is ok now? When you say speech problems, do you mean he struggles to find the words or that his speech is indistinct (could he have bitten his tongue, if so?)
It's obviously concerning you, so I think perhaps a phone call to 111 might be sensible, just to make sure it's not something unrelated to the hypo.