I'm sorry to hear about RooRoo. I'm Joanne and I have 2 paralyzed dogs, a Lab and Tiny a mixed breed. Abrasions and ulcers are something that I am on the watch for all the time and they can, as your son is finding out, be difficult to heal. At this point you might want to have your son talk to his vet and let them know that the wound is not healing. They should probably have it cultured as it may well be infected. I unfortunately have had to deal with ulcers where I could see tendons and bones. These wounds typically heal with regular cleaning and bandage changes (twice daily), although sometimes need antibiotics and, in severe cases surgical debridement and closure.
Wishing you the best