|Unfortunately that is very common. The rash will break out FIRST on the areas that have thinner skin and/or more oil absorbed. Since the oil cannot be seen on your skin when you are exposed, you really have no idea where you are going to break out, especially if it was on your hands first. For example, a person could ONLY have it on your hands initially, but then they touched their face or gentials and they unknowingly spread the invisible oils on various parts of the body.
It is too late now for your son, but for other readers: PLEASE DO NOT TAKE A BATH IF YOU THINK YOU HAVE BEEN EXPOSED TO POISON IVY OR OAK----TAKE A LUKEWARM SHOWER INSTEAD! The bath water can easily spread the oils to all parts of the body and cause a much larger rash! If you tub-bathe after the rash has erupted on the skin, then it is okay....by this time the rash cannot be "spread". ALSO: DO NOT TAKE HOT SHOWERS! People on here say it helps with itching, and it does this by temporarily tricking the nerve endings to feel tlike they have been "scratched". This is a BAD IDEA regardless: The hot water opens up the pores and causes the oil to penetrate deeper into the skin cells. Your recovery time will lengthen using hot water to manage itching. Use VERY COLD water compresses. This method is excellent for itch relief and recovery.
I would recommend Zanfel for your son and friends to wash themselves with. It is very effective and it could help with his outbreak even though it has been a while since his initial breakout. Since Poison Ivy rashes can last a good month before completely healed and he has a severe case, I think it is worth the cost. It is easy to find. Lastly, expect a rebound rash from the steroids. Prednisone suppresses the immune system. The oil irritant is still there in the skin cells, but the body is being "held back" from responding when taking steroids. That is why it appears as if the steroid is making the rash go away. Zanfel actually helps remove the oil from the skin and is a better option. Check out the Zanfel website for more info & pics. Read my posts here on Prednisone so you will know what to expect from steroids taken for rashes. Best of luck to everyone.