You dont mention how old the dog is?
A dry nose is normal in dogs unless they are presenting with other symptoms such as diarrhea or vomiting then it is time for the vet.
Believe it or not dogs can loose energy when they are bored.....as well. So if the dog is not stimulated but is eating and drinking ok, he could be bored.......just a thought. OR over exercised it works both ways.
BUT for me if my pooch was normally an energetic dog and lost energy I would wait 24 hours just to see if she perked up as sometimes dogs just want to laze about a bit...then I would seek the vet.
HOWEVER, if the dog had no energy, was not drinking or eating I would not wait 24 hours.
I hope your poochy is fully vaccinated for all the known dog diseases as prevention in these diseases is really important especially with Parvo being so virulent in dogs now.
Is this a sudden thing? For instance, does your dog usually have good energy and want to go walks and play? Does he eat & drink well usually? You say he's eating ok, and drinking...some....but the sudden lack of energy is a sign he doesn't feel so well.
If he's suddenly got like this, or has become worse recently, you need to take him to be checked out with a vet.
Dogs can, from time to time, have dry noses, and be fine. That in itself, occasionally isn't always a sign they are sick. But with no energy....that isn't good. I can't tell you, obviously, what is wrong with him, but he needs to see a vet.