Poor boy..well poor all of you! My daughter, who is now 18, has such a sensitive bottom, that she would get yeast infections and the rashy part you describe. Disposable diapers became her worst enemy for days on end..well any diaper really, but they made things infinitely worse, so we went on cloth diaper benders, medicated creams, yogurt slathering, whatever was recommended by anyone at that point. This started when she was very young and went until she was finally, reluctantly, potty trained a few months before she was 3.
As she got older, she would hold her poo as well, but it was always a losing battle for her as it was not solid. She had sensitivities to foods that were not consistent from month to month, so it was hard to track it. Ultimately it was just her system as she still has some struggles today (she is 18 now).
Things did not show any improvement to this ongoing issue for us until she was potty trained. It really was the first time in her life we had a better handle on the raw, open sore skin that her bottom would become from one minute to the next. She could be fine for weeks, then BAM, we were in booty hell for the next 2 weeks again. Then it was trying to control her diet, watch for triggers (she never did learn to like the taste of milk until she was at least 10 and we reintroduced it to her), and do things like no bubble bath, only sensitive skin items like laundry soap (no dryer sheets or fabric softner) and only cetaphil lotion, etc. She never liked the idea of sleeping bare bottomed, but we did try that as well.
I wish you luck...its awful to see our children in pain....
EDIT~ I do home daycare and actually see much of this excessively bad erosive rash pop up in kids, where they become bloody and sore for days from one change to the next. Many get an ointment prescribed by Dr's called, elegantly enough (on the prescription!!) POOP GOOP. Apparently it has Maalox, I think Nystatin and something else maybe in it? But it works like a CHARM!! Maybe its a local thing, but I got it for my daughter, and now current daycare kids still get it. Sometimes its thick and pasty, other times its more runny, so maybe each pharmacy makes it different? But it helps a
lot!