Sounds like hypoglycemia/low blood sugar.
Try and get yourself some Nutri-Cal and give it to him once a day. Could be more I'm not sure I don't have the bottle sitting in front of me, but I give Teddy and Lola that once a day and they love it. It's a high calorie dietary supplement that provides nutrition in a gel form. It's great for puppies who are finicky, off food or require an additional source of energy.
I hope everything turns out ok with your boy!
