L.S.
I would think that the oil from the poison ivy can not stay viable on the skin for that long. especially after showers etc. Are you sure that he is not constantly coming into contact with something that is still covered with the oil and thereby coming in contact with him? Something he might not realize, like a baseball cap that has not been washed in a while. Or a cat or a dog.
My son had poison oak on his face and neck, ONLY, nowhere else. I couldn't get the rash to heal. We finally went to the doctor for antibiotics and steroids. It finally went away. We think it was the cat. His cat. He rubs and nuzzles that cat daily, multiple times a day. Gave the cat a bath. Never had anymore.
I would think that if something keeps coming back and he is not reinfecting himself with the poison ivy, then the "something" is likely from a different source. The areas that had the ivy rash maybe more vulnerable to the other parts of skin, since the integrity was breached by poison ivy. But because it was irritated, it might be a secondary bacterial infection that is just lingering in a susceptible spot. The infection may look to get better with ivy treatment, but doesn't completely heal because the ivy treatment is not the treatment for an infection. Or it could be eczema also, not poison ivy. Some other types of skin conditions look like poison ivy.
I would go to a dermatologist for some advice and consultation. In the meantime, I would use some OTC hydrocortisone cream or oint, and some antibacterial ointment. The bleach solution would work, but honestly I would dilute it first. It is pretty caustic to the skin, especially broken skin. And it will burn, and cause irritation because it is a chemical. But truthfully, sterile bleach solution is used as wound care treatments. It is called Dakins Solution, basically diluted bleach, sterile. We(as in medical personnel) use it on stubborn infections by soaking the gauze in the solution and packing the wound with it. But I wouldn't use the clorox full strength.
Dematologist if all else fails.
Good luck,
L.