Just wanted to update: he is doing well. Off his meds now and no change in behaviour. If anything he seems more affectionate than before the neuter. Spraying behaviour is abating. He explored his kitchen litter box before peeing on the floor... a step in the right direction, I guess (in his cage, he's perfectly litter trained!)
His wound looks good! My vet suggested it might be a seroma and to keep an eye on it. It has kind of dried out now, and may slough off eventually with new skin underneath. No sign of infection! I think I'm out of the woods. Phew!!!