I was told on here if he's young ( 12 weeks or so) give him nutri-cal or if you don't have that you can give them a 1/2 tsp karo syrup, so they keep their energy up and don't have low blood sugar. I had to do that to Tyke the first week we had him and that was when he was around 12 weeks old. It helped alot.
Also Have you changed his food lately? I had to go buy the same food the breeder was feeding Tyke before he would eat. And I tried 5 kinds of food. So I called and found out what kind she fed him and when i went and bought some........well he hasn't turned it down since and that was back in Sept.
yeah i seen in your profile that's he's 2 monthes old....you might have to use the karo syrup to help him through.
hope this helps some.