S.D.
I know that often people will tell you that infants don't need water but I disagree because of living in AZ. One of our family friends is an OB nurse and she told me that giving a baby water can't hurt them. So I don't think it can't hurt to give your son some water. We started giving our daughter water very early on (she was born in June so it was super hot). If he's not interested, I would agree with adding some juice but not too much because of all the sugar. The straw trick is great too because they think it's "fun" to drink out of the straw. And of course as Fawn said, watch for the physical signs.
And yes, keep yourself hydrated as well. Pretty soon you'll start to notice that most people in Phoenix don't go anywhere with out a bottle of water. I know we sure don't! Good luck and welcome to AZ!