How do I calm my partner’s jealousy from afar?

So my partner and I have been long distance for about 8 months now we’re usually solid but lately things feel tense. There’s this guy, Mark, I work with—he’s been one of my closest friends for years. We text a lot, mostly about work stuff or dumb memes, but also personal stuff sometimes. I’ve known him longer than I’ve known my partner & honestly he’s just always been there for me. My partner found out we had lunch last week (it was a group thing but still) and kind of lost it. Said it feels like I’m replacing him or whatever which isn’t true at all. I keep saying Mark’s just a friend but he doesn’t seem to believe me. He’s never acted jealous like this before & I don’t wanna cut Mark off—he’s been through my breakup with my ex, job stress, everything. I get that distance makes trust harder but this feels different. I’ve sent pics, included him in convos, tried to be transparent but he still seems insecure. Part of me wonders if he’s comparing us to something that happened in his past but I don’t even know how to bring that up without making it worse. How do I make him feel secure without losing a friend who’s been like family