Poor kid, my grandson is 11 and still has accidents. The guilt they can feel from this is horendous. You have to stop making any kind of an issue out of it. He is internalizing it and he will hear your words over and over for the rest of his life. You have to make sure he is showering each and every morning, I told my guy that there wasn't time in the evening and I needed him to plan on bathing in the mornings. I told him I really appreciated him making it possible for the little kids to have their time in the evenings. Have a hamper in his room and just check each day like you are but I think I would just have to accept that this is horribly embarressing to him and let it go, of ocurse he is going to try and hide it. Mine can't hide it, he rolls up in the bed coverings and is kind of "burrito style" so everything is soaked, even when he wears underjams, they don't work well as far as I'm concerned. Overnight pull ups worked much better until he out grew them, dang it!
Keeping fluids from him won't do anything except make him dehydrated. His brain isn't getting the signal to wake up and go potty. It may be reducing the amount he's peeing but it is much more concentrated urine and that can contribute to more issues such as UTI's and Kindey stones, see more below.
We had ours to a Pediatric Urologist and he had a few things to add but basically it's wait and see if the connection develops at this point. There wasn't anything biological. There are meds that the doctor can give too, new ones come out all the time but who knows what long termeffects they'll have.
Our Urologist did make a good point though. If you put a drop of water in a gallon of sand what do you get? A tiny grain of a rough, scratchy, concrete like material that is difficult to pee out. If you put a grain of sand in a gallon of water what do you get? You still have a gallon of clear drinkable water. Restricting his fluids is increasing the possibility of kidney stones. All food and drinks have minerals in them that don't dissolve in the body and "rinse" out. Let the doctor give you some input on this topic.
Hope you find a solution!