Is it safe to routinely give an infant water during the summer time?

A baby his age can safely have limited amounts of water between bottles, sounds like your pediatrician has already covered that with you. The formula will hydrate his body to an extent but it is sort of like adults eating watermelon- there is liquid in there but it is also filling so if you're only thirsty you're better off just drinking plain water. If he's sweating then it's best to just use water between feeds.