Needs labs: consider check adrenal glands
My older dog has no appetite in hot weather, will only eat at night, and not that much. Perhaps thats your dogs problem.
If the vet has examined him well, and can't find any problems, then the only thing you can do is to wait for the results of the blood test, which should show up anything underlying this.
It is just possible he has a slight tummy upset. Perhaps nothing bad enough to make him obviously sick, but enough to make him feel 'off' and put him off his food. Then if he's not eating as usual, his energy levels would be lower.
Just a thought, but is the weather very hot or humid where you are? Because if it is, he may not feel like moving or exercising, and that would affect his appetite as well.
Another thought...has he had any changes recently, or any upsets, like you going away for a day or so...or anything similar?
Most likely it's nothing like that, but those sorts of things can make a dog behave differently sometimes, and can occasionally affect appetite