My dogs nose isn't wet... Help!?

Okay now my dog is 12 years old and was diagnosed with liver problems just a couple of weeks ago. The vet also said he was dehydrated at the time. He is only allowed to eat spelll food from the vet, all other food will kill him. He has to take 3 tablets a day which makes him drink and wee a lot. He does have diarrhea as well. This morning he kept me up from 5am-7am, I let him out for poos and wees. I then realised his bowl of water was empty and I wasn't sure how long it was like that so I filled it up and he drunk quiet a lot. I also feed him some of his special food. I then woke up at about 10:30am and he had a dry nose. I let him outside and he came back with a wet nose. But it was then dry again so I have made him drink a lot today and it goes wet for about 10-15 minutes but then goes dry again. I am only 16 and my parents are away on holiday so I can't take him to the vet. I'm pretty worried. Can someone please tell me why his nose is not wet? Thanks.