I doubt that anything is going on other than delayed disappearance of posion ivy. If exposure was extensive (and it would have come from the bushes, not the ball, then ongoing appearance of new lesions can go on for days to weeks. These new lesions are due to continuing immune activity, not spread from posion ivy resin on the skin, which has surely been washed off many times over by now. It is possible that the early doses of prednisone were too low to suppress immune activity. It is also at least conceivable, though unlikely, that the original diagnosis was something else. I therefore think that dermatologic referral makes sense. It certainly doesn't sound like scabies. So go ahead and get another opinion, but I am confident that at the end of the day, your son will be fine. He'll just need to be carefl in or near the woods.
Best.
Dr. Rockoff