Hello & welcome to Just Answer! If you have any questions after reading my answer, be sure to let me know.
I am sorry to hear about your son's problems
Is he having the same kind of rash again?
Can you describe the rash if possible?
The most common cause would still be some sort of mildly contagious viral infection
& this unfortunately takes its own sweet time to become better
However, resting a lot, plenty of fluids, and taking a good quality vitamin supplement may help in recovering sooner,
If the symptoms continue to recur then evaluation for rare causes of conditions like Periodic Fever syndromes and immunity related problems may be warranted,
BUT not at this point of time, in my opinion,
Please get back for any clarifications,
Do leave a positive feedback if the answer was helpful,
Dr Gupta, MD