1 yr baby having red rash only on arms,ped suggested mosquito bites, but i am not convinced.can it be allergy due to hot dump weather?how to treat it?

Red rash: Has your baby gotten a new shirt? If so, he might be allergic to the material or to the finishing product used on the shirt. For now you can use a moisturing cream like Eucerin or cetaphil

Followed peds advice: I would trust your pediatrician since he saw the rash. If it is a heat rash, there really isn't any treatment other than prevention by keeping the skin cool.