Hi and welcome to HCM
from your description it seems your son has allergic tendencies, with
allergic rhinitis or colds. It usually is present from childhood. They are usually prone to certain recurrent skin allergies.
It could be a case of eczema aggravated due to certain allergies he was exposed to, but it is unlikely that it would start at his age and not earlier.
Try to remember if he was exposed to any insects, dust, plants,pollen, any applications on the area or outdoor activities, which could have caused the rash. is there any fever or similar complaints in the family or contacts.
Another diagnosis to rule out is
psoriasis as it is usually present near joints and takes weeks to get better. It will require good treatment.
I advise you to
consult a dermatologist to get the exact treatment, and also to rule out any infection.
For now you can use antihistamines like levocet or
fexofenadine daily. Apply a
mometasone ointment which can treat both . Use a mild soap. If allergic, it will subside early with the above treatment.
Do consult the doctor as advised if you see no change as it may require longer period of treatment.
Hope this helps.