Picked up dog after boarding for 17 nights, he is thin and appears to always be hungry and thirsty?

For the first time in his four years of life, we left him to a boarder while going away on a trip. When we picked him up he lost a significant amount of weight, from roughly 25 pounds to 20 pounds. He got home, was excited but has been drinking a lot of water and appears to always be hungry! Before boarding, he used to be a picky eater, not drink a lot of water, and would rarely finish his kibble, but now he cleans up the whole bowl and even appears to ask for more. It's been 2 days now, is this normal?