Hi,
You could just apply some calamine lotion on it to soothe irritated skin.Also give him Benadryl liquid to counter act any possible allergic re-action.
To prevent further bites
you could keep him fully covered, and also eradicate the mosquitoes by taking some house-hold measures.
IF it seems to be secondarily infected, you could apply an anti-bacterial ointment meant for children.
They will usually disappear in a week's time.