If he's eating wet food, he'll be getting most of his water needs that way. Cats aren't big water-drinkers unless they eat dry food. They're also notoriously fussy about what water they'll drink, particularly with regard to chlorine/oxygen content, although - being cats - will break the rules when it suits them. One of ours would only drink water that had been standing around for a while (plant pots and toilet bowls were preferred to the water bowl), unless it was a non-turbulent trickle from a tap, which she'd lap in mid air.
I'd suggest offering some water that's been boiled and allowed to cool, and possibly a variety of containers, to see if he takes an interest.
A quick google confirms that you can do a skin pinch test for dehydration on the scruff of the neck, much as you would for humans.