Hey plowter!
Glad he is ok! :)
Tuna juice to my moggies is literally the juice from tins of tuna! Whenever I use it for my dinner, I always squeeze the juice from the tin into the water bowl!
My lot love it, and for them it is very diluted, as I have 7 cats and their water bowl is actually a casserole dish!
Also, one thing to give him as a treat to try, is a raw egg egg and a tin of tuna mixed together! My lot go nuts for it, may not help with him drinking water, but it goes down well!