Wow, what an undertaking. Bless you for taking him on. I've not got much experience with rescues, but do have some oldies. At his age I'd wonder how are his teeth? My oldest is 30 and is very lacking in the teeth department. He is an easy keeper but only if fed a mash & quality (no stems) hay that I chop. He just can't grind and gets nothing out of pellets or his hay if fed straight.
Your vet will definitely be your greatest resource in this. He could have underlying problems that need addressed before any safe advice on his care could be given.
I'd second the kool-aid or electrolytes. Mine are spoiled drinkers (our water is spring fed and tastes like bottled) so when we go other places they most generally refuse the water. I buy the generic diet sports drink powder (diet because of the sugar content) and add a spoonful to their buckets.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.