I need some advice.

I hope this is the place for some wisdom.My brother came out to me about a decade ago. At first, only my mom and I knew. Eventually he told my dad who was disappointed. He is pretty religious and more prominently, had an image of what his sons would become in life, with gay not being a part of it. So there was a sense of disappointment and internal struggle on my father's end.My brother and father love each other but it's put a serious strain on their relationship. I can see my dad trying but it's hard for him to reverse 60 years of thinking a certain way.My brother is at a point where he wants to propose to his boyfriend. My dad's side doesn't even know (theyre also pretty religious) he's gay and he's scared when they find out he's not only gay but also ENGAGED, it'll change everything. The great relationship he has with them.He reached out to me with phone calls and I don't know what to tell him other than be brave. I don't have gay friends to tell him "this was their experience". I feel like I can't help my brother overcome his fear, and I feel weak in not being able to.I need some advice on what to tell him.