A change in behavior can be a signal of an internal issue, but it's also possible that the dog days of summer have just hit him and he's tired. As long as he's eating, pooping, etc, and there's no unusual nasal discharge, sneezing, coughing, straining to urinate/incontinence, I wouldn't worry about it just yet. The mellowing out process seems to take quite a while--it doesn't happen so suddenly--so I am more inclined to think it's a reaction to the heat or him picking up on your nervousness.
I'd mention it to the vet when you bring him in for the neuter.