Hi D.,
You son doesn't "need" water, unless you are on a bf schedule and you don't allow him to eat when he needs to. I allow my son to nurse on demand because I am his only source of hydration. I know some people don't believe in letting a bfing baby "snack," or they think he's using you as a pacifier. When in fact he may be thirsty and need a little drink, so don't be surprised if he nurses more often for shorter periods. I live in a hot area (it's been over 90 the last few days, and they say it will be over 100 on Tues), and on those days I drink extra water and make sure that he is nursing too.
I find that my boy will not eat when he is hot, so make sure he stays cool so he can feel his hunger signs. I rinse his feet and legs under the sink with cool water and get his head wet with a cloth. Then he cools down and is ready to nurse.
If you feel he is dehydrated, you can try wetting a terrycloth bib or washcloth and refrigerating it and he can suck on the bib and suck the water out. I do that for teething, but it would work for hydration.
There are lots of support groups for nursing moms, and they have educators who can help you. Sometimes doctors are not so pro-breastfeeding, like they don't trust it. Not really sure why...so if you need information let me know. :)
Good luck!