I am sorry your boy has this issue. He may have a cold coming on like canine flu.
Feel all over his body and look in his mouth for any abnormalities or evience of pain.
His temp should be 100 - 102.5 to be normal. Any lower and he may have an intestinal problem.
A dogs appetite is the best barometor of his health so eating good means he feels well.
Net, we check stool and urine. That seems fine from what you have told me.
His gums are normal so we can rule out anemia and probably worms.
See if he will let you take his temp.
Add unflavored pedialyte to his water 50/50 to hydrate him and balance his electrolytes.
Put a few drops of pancake syrup on his tongue to raise his blood sugar in case he is a bit hypoglycemic. That should perk him up.
Also give him one teaspoon a day of plain yogurt to keep "good" bacteria in his system.
If he develops any abdominal pain, bloating, constipation or cannot keep anything down he needs to be seen by a vet.
Please let me know how he is doing. I will always be here for you
I hope your boy feels better very soon..
Best wishes for him,