good old baking soda
Use enough water to turn the baking soda into a paste. Dab this on the mosquito bites. It stops itching almost immediately, and helps to "draw" the poison from the skin. In the days of old this was called a poultice.
You can use this however often is needed. If the poor little guy has alot of bites on his body, you can soak him in a warmish bath with a good handful of baking soda, as well.