I understand. My older daughter has to go to speech therapy, and she rarely says anything. Plus she hates sitting on the "potty". She used to be okay with sitting there, but now she hates it.
Go commando, pants feel too much like a nappy and they think it is.. we did a whole month commando (when I say “we” I mean my child 😂) and then started using pants when she was potty trained.
It’s such a hard process but you are doing fab, maybe set him an alarm for every 20 mins and take him every time it goes off, then gradually up the timer. also does he have a toilet card that he can show you when he need to go? With the underwear maybe just let him be free for a little while until he’s used to it then introduce underwear once he’s got it. Not ideal I know but if it works.