Don't let him have toys easily accessible to him and make sure that they are the ones to give him toys. His good, fun, special rewards need to come from them for a while. Have them put him in a sit and give him a toy for a reward. Have them feed him but put him in a sit always. Let them brush him and give him treats. If he sees them as his competition he will growl to warn them away from what he thinks is his. Nothing in your house belongs to him so don't let him have something without being given something.