skin rash

My 17 month old son developed a rash that started on his arms and back. It has now spread over his entire body. There is not one spot that has not been affected. It is red splotches that are starting to get lighter centers. They are raised a little but not much. It doesn't seem to bother him. He hasn't had a fever and there's nothing in his mouth. I've not changed anything in his diet, the dtergent I use, the soap i use or any chemical in the house for cleaning. His half brother developed a rash 2 weeks ago but his went away in 3 days. My son has had his now for 1 week now. since 3-30-09. It is only getting worse. I took him to his doctor and he said he'd never seen a rash like this in a child his age and had no clue what it was. he suggested not doing anything for it. Can you please help and maybe give me an idea of what I can do to help my poor little guy? I have added a picture of one spot of his rash. Please email me back. my email is ***@****