
I have never
ever heard of a dog being circumcised - as they don't have a foreskin (only a sheath/prepuce) I don't believe it would be possible anyway! His sheath may be tight, but it shouldn't be removed!
Character change like this is almost always down to a medical cause, usually pain. If he is an active dog it is likely he has pulled a muscle (possibly in his back as he is a long-bodied breed) and this is what I would first suspect. If rest and possibly (mild) pain-killers don't have an effect it would probably be a good idea for him to visit a chiropractor if possible.
But I doubt very much if it is anything to do with his sheath! If the vet says that's okay, leave well alone! :)