hmmm! i do find that they all differ in their water intakes, some drink next to nothing, some drink more but i guess five chins drinking a third of what one chin drinks would be a bit worrying. how long have you had him for? is it possible he was kept in a colder environment and he's adjusting? they tend to drink more when it's warm (i'm not suggesting it's too hot, just that perhaps it's warmer than he's used to)
how old are your other chins? i don't remember if kits drink more but that may be it? is he putting weight on?