It has been known for males to act up right after they are neutered. Some that have never marked or humped will start doing it right after they are neutered. I believe it is caused by the hormones that are out of line but it usually only last a few weeks and then the dog goes back to his normal self.
This may be what is happening to your boy, only time will tell. Hang in there in the mean time and make it clear to him that his behavior is not acceptable. |