I have this with dd1. she is 3 and ASD, and for most of her life has refused to drink.
We tackle it mostly with diet. so, she has porridge for breakfast (absorbs loads of milk, then an Organix fruit pot added (they are mostly water anyway)). Then she has some cereal, again with more milk.
she has all foods in sauces (every drop counts!) so has soup for lunch, or spag bol, curry etc - anyhting where I can water down the sauce to get more flluid into her. same for tea.
Have you tried extra custard etc? rice pudding? jelly is another good one - it's obv mostly water. And the Alpro soya desserts, and fruit puree pots designed for babies - all good by way of water content.
will your ds eat fruit? if so, then add in grapes, melon, oranges - all great for staving off dehydration.
dd1 is at the moment (touches wood frantically) drinking again, but this is a maximum of 200ml fluids in 24hrs. The rest she still gets form diet. She has just come out of a period of about 9 months where she did not drink a single drop at all during the day, and she has never yet been dehydrated - we have managed this through diet.
If you search my name, you will come up with a coulpe of threads from last summer with some good advice re: drinking.
I hope you find a solution. It is a huge worry when they do not drink. I think what helped us enormously was us abcking off totally, secure that dd1 was getting enough fluids from diet, and slowly, slowly, dd1 came around to drinking again. It has been a long fight.