Hi
Thanks for the post!
The rash could be measles, chicken pox, allergy or herpes shingles. However since the rash is only in one side I feel its most likely a shingles or eczema or allergic dermatitis to something that part of the body came in touch with.
I think you should discuss this with your pediatrician. It is difficult to comment beyond this at this stage without examining the rash. Please let me know if there is any thing else and do keep me posted. Take care!
My son also had a rash that sounded similar to your sons, my son was a little over 18 months when he had it. It turned out he had shingles. He had to go see doctors from infectious disease, they said he got it basically from having the chicken pox (varicella) vaccine. Because it is a live virus, it lays dorment forever in your body, when it get reactivated, it is shingles. We had to see 3-4 different doctors, all of them agreed that it was shingles, and all said it was extremely rare in such a young child. Many doctors do not have alot of experience with shingles in toddlers, if you have a childrens hospital nearby, i'm sure that have a pedatric infectious disease.
My son actually got it twice, the first time was the worst, it lasted over a month, and he seemed to be in pain quite often. The second time it lasted a little over a week.
If it is diagnosed early enough, they may start him on an antiviral, which may shorten the duration and severity.
Good luck to you and your son