[yorkietalkjilly]

I would have him fully checked out by a vet to be sure this behavior isn't health related. Behavior changes are one of the first indications that a dog is ill or in pain. Not just a physical exam but a full blood panel and heartworm test and fecal. Then, if those are all normal and his neurological/full system physical exams are normal, you can look at what is causing this behavior.

Dogs seldom suddenly become aggressive with humans unless it is medical. If it is a behavioral issue, likely it's been building for up a while, with the dog losing some respect for his leadership, guarding things he considers should be his a little or becoming possessive and it's just things you've overlooked or let go. Fear, frustration, stress, trying to achieve dominance with what the dog might see as somewhat weak leadership can all contribute to this. Starting him out with the Nothing In Life Is Free program where he has to obey a command to get even his basic needs met can start to show him who is really in control and the dog enjoys the process. Further enrichment of his life with more walks, challenging games and x3 daily obedience training daily in short 5 minute sessions, where he learns how to do what you say quickly for praise and a reward, all serve together to meet the basic needs of a healthy, happy dog, fill them with achievement and all of the busy activities and and teamwork teaching them self-confidence and building you up in his eyes as an authority figure to be respected. A rehab program like this can help defuse aggression at first and eventually just wipe the tendency out.