Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. There is a good possibility that the way you describe his rash it could be eczema. When someone in the family has eczema, usually there is another family member who has hay fever, asthma, or eczema. Infants who have eczema typically have their red rash on the face. However, older children, like your son, have a rash behind the knees and in front of the elbows. A hallmark of eczema is that the rash is very itchy. This does not have anything to do with his ADHD medication. Should this be eczema, you were correct in placing him in a tub of lukewarm water for 5-10minutes. Take him out of the tub, pat his skin to remove beads of water. But keep skin moist and immediately apply a moisturizer to hydrate the skin and prevent evaporation. You may try Aveeno cream CeraVe cream, Eucerincream. The anti-inflammatory medicine to use can be Hydrocortisone 1% cream(this is over-the-counter). Apply to the surface of the rash two times a day. If this is eczema, continue to moisturize his skin even when the rash disappears. If this treatment plan is not effective, then your son needs to be seen by his primary care physician. Hope I have answered your query. Let me know if I can assist you further. Regards, Dr. Arnold Zedd