What are you meaning by lethargic? Like he acts sick, or he is just lazy and finally acting civil
If i dont work my dog he is smitten, zooms around the house, barks at me to play and go outside, but afterwords he is civil, peaceful and mellow.
How old is the dog? all dogs are different, some dogs dont take as much to work as others will take all day before they tire.
Def. needs to get use to it since its been lazy time for the last few months.
For the water thing, give him some water to chug as usual, but not as much, then put icecubes in the water dish to replace the rest of the water. This way your dog doesnt have a chance to drink himself into vomitting. You can also do like a 50/50 mix of water and ice cubes.
Hope some of this helps.