I have a son 3.5 years old and i just noticed all over his body small red pimples, some of them filled with water. what it wold be? and what should i

Chicken pox: It's possible that this is chicken pox. Is he itchy? Fever? With varicella, there are 3 kinds of spots- 1) flat red spots, 2) clear blisters that open easily if you brush them, and 3) broken blisters that start to crust over. Usually, there will be all 3 types within the first day or so. Tepid baths with 2 tbsp of baking soda can help. Call the doctor if there is a high fever or other symptoms.