C.K.
It would send up big red flags if any of my children responded that way when their dad or a family member walked into the room. My babies/toddlers were very much attached to me, but I can't recall a single time they ever reacted that way (if I was still there....now it was another story if I was trying to leave them in say the church nursery!)
Has your fiance ever been left alone with your son, even for a few minutes? If so, I think it's very important to consider the possibility that he hurt and/or scared your son. I realize that's a harsh thing to say, but I have to be honest, that's the first thing that came to my mind.
As for not being on the same page parenting-wise...he is NOT the parent! He is not your son's dad by birth or adoption AND he is not your husband, so at this point, his parenting philosophy doesn't enter into the picture. I agree with the person who recommended moving back in with your grandparents. By living with your "fiance", your dependence on him can't help but influence you and possibly cloud your judgement, making you second guess your mothers instinct. Living with your grandparents sounds like it may provide the stability you both need while you get back on your (own) feet. I wish you the best...