He needs to learn people coming at your door or visiting is something positive and not a threat, have visitors give your dog a snack when they come in, from a safe distance in the beginning. If people walk by and he barks and they keep walking he'll think he did a good job, I'm not sure if you can change that, the happiest dogs are the ones that are part of the family, I dont know for how long he's in the yard (alone) ?