Has he had a full medical check up? Especially a full Thyroid panel done? I really would doubt that his being neutered has anything to do with it… certainly I have never heard of that before.... but I would first get a full medical done (including an eye exam with a Ophth), with full blood work and full thyroid panel (which typically must be sent out separate from normal blood work that your Vet might do) and then contact a behaviorist to work with him (and the family).