E.E.
How does he have mosquito bites in February? Mosquitoes die in the cold weather. They have to be some other kind of bug. I would take him to the dermatologist if they are lasting for 3 weeks. That's not just a normal mosquito bite. It is infected or something else is going on.
For the vacation, you can use Bite Blocker for Kids, which is a soybean oil based product without any chemicals in it. Here is the company website. http://www.homs.com/