Why does my JRT try to guard my elderly father?

by mary
(chicago, IL USA)

I have a JRT, a neutered male 4 yrs old. He has begun viciously attacking me at bedtime and keeps all members of the household barricaded in their rooms until I give him a treat. Once he has the treat all is wonderful, he sleeps with me and is a perfect darling. He seems to try to guard my elderly father's room and will not allow me to enter his room. If I try he attacks with a vengeance. We are not talking about nipping here. Had I not protected myself with a pillow he would have ripped me to shreds. Help!