All these symptoms with similar history in other kids is suggestive of some viral infection.
Maintain adequate hydration by giving plenty of fluids. Also give him Zinc syrup. This will help in better epithelialisation (regeneration of cells of respiratory, gastrointestinal tract and also in recovery from skin rashes).
Apply some Zinc oxide
based cream over the rashes and observe its course over a period.Also observe for signs of dehydration
. Most of the viral infection with these symptoms subsides on its own. So no need to worry but it will be better if you consult your doctor for proper evaluation.
If rash increases or there is pimple like lesions in between rashes consult your doctor immediately..