Thanks for posting your query.
It seems like an exanthematous rash
which occurs commonly with viral flu. I do not know if he has fever too along with this.
In most infants and children the treatment is symptomatic. You can give him anti-syrup containing cetrizine
for the lesions to subside. Also, apply calamine
lotion to the affected areas topically.
If there is fever, also give him acetaminophen
He should be fine in a couple of days.
Hope this helps.