You're right to focus on preventing the bites from occurring. I would start off by avoiding outdoor exposure during peak mosquito times: dusk is when they tend to come out in large numbers. Plan to do some sort of indoor activity during that time. The other thing is to try to have him wear long pants and sleeves. This can be difficult to do if it is very hot, though if you use light cotton clothes it is often quite comfortable.
You do want to be careful about using insect repellents as the active ingredient, diethyl toluamide (DEET), can be toxic if absorbed through the skin in large amounts. Definitely use products that are made for kids, with no more than 15% DEET in them. If you use a stronger product, consider spraying it on the edges of the clothes. It will keep the bugs away but won't be touching your child. Remember to wash the repellent off of his skin when he comes back in the house.
If mosquitoes inside the house are a problem and you can't seem to get rid of them, you may want to consider getting some mosquito netting to put over his bed to prevent him from being bitten at night.