Sarah, what's the weather like where you are? If it is really hot, it could be that it is the heat affecting him. A lot of people (young, old, and inbetween) struggle with the warmer weather - especially if it is a sudden hike. They tend to lose their appetite, thirst, and complain of nausea (which decreases their desire to eat and drink). If he perks up in cooler temps, then I'd say that could very well be the culprit. Pop him in a cool bath or the pool and encourage him to drink whenever you can. If it persists by the time you get to the GP for your appointment, I'd def get him checked out.

HTH

MG

Edited because DD hit the send button!