Regardless of whether I'm a rabid rabbit (yeah, I go on a bit, if that's the complaint...
) I think I better reiterate my later ideas about what I
think is good advice.
Given that you weren't the absolute bestest friends that I'd originally read into your original message (well, assumed, really... from your general concern, but you'd not really said as much), you're not going to be this guy's main contact during this portion of his life where he will (hopefully) undergo rehabilitation and at least partial recovery.
So you certainly don't need to be continually guiding them through life. You just need to be yourself when you happen to be in contact.
He'll have met any number of people (i.e. everyone he
has met, since the episode!) that he doesn't remember. He should know by now that he'll be meeting people that know him already, to one extent or another. And, from your later info, I know you've actually spoken already. So there's no benefit in now going back to "as if you never knew him". If it isn't taken as an insult, it'd be just a cause for confusion. (I'm assuming he's not got a rolling amnesia which means he still forgets anyone he met the day before, of course... But even then there's no reason to 'roll back' your attitude... Even if he doesn't remember people, he'll remember that there are people that he just happens to have forgotten, I would imagine.)
It'd be tempting to reshape things "Yeah, you
loved to go and see <your own favourite sports team>..." when his favourite sports team was actually their rivals and this was a major bone of contention between you. Even if other people's comments and corrections don't trip you up, I can't see him being helped by being in the wrong set of supporters' stands if/when you pursuade him to support
your team in a game against
his old team... And that's just a flavour of the kind of wrong-footedness you could create.
Obviously you can't ignore something has happened. Awkward moments as things of significance from your shared histories are talked about in front of him, by you or others. But he'll be used to it. (Or be getting used to it.)
Again, imagine an old acquaintance came back into your life after having an arm amputated. If he wants to joke about how he's "mostly 'armless" now, or whatever, let him. If he needs help to crack open another beer, or something, then help him. But staring at the stump or horrible first-generation prosthesis isn't going to do any good, nor is obviously staring
not at the stump/appliance. If you think he'll take to the new nickname of "Lefty" with good humour, then go ahead, but you'd sound it out first before going with it over whatever you always
used to call him... But (in this analogy) if you're again not in his prime circle of friends, consider that he's probably heard a lot of the jokes before, and maybe you're not the person he wants to hear the name "Lefty" coming from, right now, even though he's generally Ok with it in the context of his closer mates using it.
Context is, of course, everything. I can't say for certain that I understand the mood around this guy, or the mood he himself is in, and as there seems to be
something about his parents' actions that concerns you it may be that there are other concerns and things that need to be worked out, now and in the future. Go with the flow? Do as you might imagine you'd want to be done by. Do the equivalent of not staring, unless he's a natural extrovert who actually gets a kick out of being "The Incredible Unremembering Man" or something, but you'd have to take his lead on that...
And I've just removed the words "and summarise" from the first paragraph, as I quite obviously haven't managed to do that...