Hello Carol,
Welcome to the forum. Well done for your successes using EFT.
I have some suggestions to help the effectiveness of your persistant tapping - Gary Craig would be proud. You may have already tried them, but worth mentioning.
If ' I hate this rash' is highly charged, then it would be an idea to keep working with it and extend some more. Measuring his SUD's or just using hands, take a benchmark of how he feels. Also you can ask him, what does the 'hate' mean to him. He will be able to give you some idea, but if he isn't then you can help him a little. What colour is it? How big is it? Has it got a name? What is it saying. Children are much better than us at using their unconscious creativity which can be so beneficial in the healing process. Give his feeling a chance to have an identity ( say the green prickly monster) and gets an opportunity to do something concrete - put it in a box, post it to the moon, etc. These type of ways of using EFT with children of all ages work really well.
Have you tapped for your feelings around your son's conditions? I can suggest that you read about this in one of my article's which I can forward to you if you cannot locate it in my previous postings.
What else is going on for your son and the rest of your family? Have you raised any emotional roots to these bad flare ups for your son. If so, these are all tappable issues. Even though he is only six, children are very smart at picking up information as you will know and they can easily aggravate these conditions.
What sort of phrase do you use at the end of the set up phrase? Make it really personable to your son - something he's really great of proud of.
Tap for really positive things in your son's life too. - eg Mummy really loves me, I was very kind when I......, I love it when......... thus helping your son to feel good about himself dispite the horrible uncomfortable feelings of his conditions.
Hopes some of this helps. Let me know how you get on, I would be very interested in an update.
Best Wishes
Christine