Agreed. If he's urinating, he's drinking some. It may not seem like much to you, but in combination with what he's getting in the hand feeding, it's a lot for that little body. Also, chins come from an arid region, so physically are capable of living with less water than we would think possible. Not suggesting that the chins we have now could survive out in the wild, but there is a genetic predisposition to living in arid regions. . .
If you're really worried about it, you could try to stimulate his drinking by hanging a bottle of 1/4 to 1/2 unsweetened apple or cranberry juice and the remainder water IN ADDITION TO his regular bottle (I usually don't go for half and half, but put less juice - it works just as well), or rubbing the nozzle of his regular bottle with a raisin to attract him. Juice grows mold very quickly, so if you do the juice thing, you're going to want to make sure to change it out and wash the bottle every day to prevent mold from growing.