Aw poor little thing! You’ve done right by him though so I hope you see an improvement.
It sounds as if he has urine scald. I’d consider using a barrier cream on his bare skin to protect it.
A rabbit I was looking after last year had this. After giving her back end a wash, we shaved away the soiled fur that was keeping the urine in contact with her skin. This really seemed to help keep her dry. Then I put sudocream on her bald areas. Her skin became a healthier looking pink within a week.
Did the vet indicate whether to wash just the soiled area? You’ll want to be careful with frequent baths as the affected skin will already be so fragile.
If you get a chance, try watch what happens when he goes to urinate. This could give the vet more of a clue as to what’s going on. The rabbit should lift the tail and urine shoot out behind the body at an angle.
If a rabbit doesn’t posture correctly to pee, it may end up running down the legs.
Some reasons for this is arthritis, spine or muscular weakness or injury. Other reasons for urine scald are infection (as mentioned) or bladder stone.
{{{Vibes}}} that he heals well.