I don't recommend Singapore. I'm not sure how that would work out, because they don't actually stamp you. It should still work, but why take the chance.
and an "N" looks a lot like an "H", especially when next to another "N". That's probably how the error happened in the first place. I'd make the effort to have it fixed but not say a word while going through immigration. If they say something, then let them know the attempt was made.
I wish I had something like that. The design of what I need to bring is so stupid. They are not at all efficient in terms of space, although I do see how explaining them would be very difficult. I'm always amused at the various special ways "authorities" interpret rules to make them make no sense whatsoever.
I travel with syringes with ice packs. They nearly always get inspected at security but it’s understandable. I go in with a positive attitude stating, “I assume you’ll want to see…” and I pass through easily (of course needing to show them because they are in metal encasements). I never offered to jab them.
3) letter from doctor including dosage and reason for medication
The latter two will only become important for medication that is either controlled or not approved in Thailand. Best to be on the safe side. Is he coming for less than 30 days then?