Go to a chinese medicine hall or even Guardian pharmacy, I believe there are at least 2-3 types of spray to prevent mosquitoes bites.
You just have to spray sparingly on the clothes of your baby (not directly on his skin) and it should keep the mosquitoes away for few hours. Repeat it say after 4-5 hours.
The purpose is to keep mosquitoes away. If they dont't get a chance to bite your son, then they can't leave any marks!