I think a trip to the vet to have him checked over is probably advisable because of his reluctance during walks as much as the change in his appetite. Hopefully it will put your mind at rest that there's nothing physically wrong with him, but if he does have a problem of some sort at least you'll know and the vet can start whatever treatment is needed.
Has he lost any weight? The reason I ask is because his lack of appetite and reluctance to exercise are the same as one of my Cockers, Sam, had many years ago. Sam was about 9 months old and became slow on walks and kept sitting down. He was eating, but not very enthusiastically. It was a hot summer and Sam still had a lot of heavy, fluffy puppy coat so for a while I just put it down to the hot weather. Then I realised that under the fluffy coat there was an increasingly thin puppy
. Sam had some blood tests and was diagnosed with a rather nasty kidney infection. A course of antibiotics sorted it out but it took me a while to get him fit and energetic again.
I'm not saying that yours has a kidney infection, but I think he should have a check up just in case there's an underlying problem. Hope he's back to normal soon