I need to get this off my chest...

So my fiancé and I bought a house, but since the house is not ready yet and still being renovated, we were living with his mother and her boyfriend. Now last week, his mother broke up with her boyfriend and went to her sister's home. So there we are, stuck in a house with my fiancé's ex-stepdad. 
So yesterday morning I was talking to him (he is 46 and I am 20😳😳😳) and he sudfenly tells me that he has been in love with me for months 😱 The thought of it makes me feel sick, I can't really sleep or eat ever since he told me this. And I can't tell my fiancé as long as our house is not ready yet, because he would definitely hit him or something. And I don't want my fiancé to go to jail. I really don't know what to do or where to go.....