Just a word from me about patience. FBF became my boyfriend nine years ago (all very traditional, met through a social group, we were mono). He hired CBF to work for him, and they became fast friends, and the three of us became great friends. When I broke up with FBF, I started seeing CBF (after my prescribed interval). Eventually, the three of of us became great friends again. For almost nine years the three of us have been friends, until we started a vee last December. None of us were actually seeking this, consciously. But I suspect the love that loves to love finds its way. I clearly loved them both, and I think they both knew it. Nine years is a long time for patience (to me). Completely worth the wait.
So, yah, helping him know there is nothing to fear is a good start. Investing in their relationship is good too.