My doctor reminds me every summer to make sure the kids drink plenty water. By the time you are thirsty, you are already dehydrated. A friend volunteers with rebuilding homes in New Orleans with the youth group from church. As a mentor they have many meetings beforehand where they are reminded about keeping hydrated and what signs to look for. If he is only drinking a glass of water, then he really isn't getting what he needs for normal activity, let alone activity for a 9 yr old. I keep telling my son he needs to drink more water. He will tell me that he just had a glass. You have to look at what he has had all day long, every day, since it can add (or subtract) over time.
BTW, when I am not drinking enough water, I get very light headed, terrible headaches and feel like I cannot breathe right. And thinking about your son's symptoms, I think I am going to really get on my son about this. He gets special medication that for the past several months makes him feel this way but the docs are puzzled as to why since he didn't have the problem before.