He may not know how to use the water bottle. Is it the same type of bottle that he had at his previous home? Some chins who are used to ball and tip bottles have a hard time adjusting to the pressure valve type bottles and vice versa.
First, make sure the water bottle is working. If it is, is it in a convenient place for your chin to get at the tip? If both of those things are fine, you can try opening a raisin up and rubbing it on the nozzle of the bottle. Sometimes the sweet smell will encourage them to lick at the raisin flavoring and then they get the hang of it.
Also, chins do not generally consume huge amounts of water. Try marking the water bottle with a Sharpie marker to see if he really is drinking and if you're just not noticing it because it's not a huge amount.
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