<human>: Can you condense the text into a few key points?

My boyfriend and I have been together for a year and three months now, and things really couldn't be going better. We've talked plans for the future, engagement, marriage, everything, and are planning on moving in together at the end of the year. (Wish it was sooner, but life issues prevent that.)

Anyways, I've noticed that he gets kind of quiet whenever I bring up my best friend of 6 years, let's call him D. D and I have never been romantically interested in each other whatsoever-we consider each other to be the big brother/little sister that the other never had growing up. We've been through a whole hell of a lot together-toxic relationships/breakups for the both of us, severe depression episodes in which we talked the other through it (I'm not sure I'd be here if it wasn't for D), family drama, the whole nine yards. I even found out that he's moving to the city I'll be moving to and I'm honestly thrilled-we've only been able to meet in person a couple of times due to life's circumstances.

I've mentioned before that we only have a brother sister dynamic, but is there any other way I can reassure my boyfriend there's nothing to be worried about or do you think I'm just reading too much into the situation? He knows that I'd rather kill myself than cheat, and the thought of cheating with D is just...ew. Incest. No. I am admittedly not the best at reading social cues.
<bot>: I think my boyfriend is uncomfortable with my best friend being a guy. Is there any way I can reassure him about the situation?