Dog Not Eating or Playing?

My dog wasn't acting normal like not eating or playing so I took him to the vet. The first time they gave him a shot and some antibiotics. A couple of days later I took him back so they gave him another shot and gave him some Centrine. So a couple days later he was still acting the same so I took him to another doctor. This time he did blood work and everything came back normal. He said there could be a female dog around that's in heat so I let someone dog sit him for 2 days. He is still acting the same. When will I see a difference? What should I do? He is not neutered. I miss my bad dog.