Boyfriend is jealous/ insecure of my friendship with my best friend (m) of 16 years

Hi! So I’m just looking to vent/ get some outside perspective. (We are all in our 20’s)

Before the summer started my boyfriend, H, and I broke up. I was dumped and devastated. My best friend, G, was there for me like he always is and when it came time to give each other their things back I had G handle all of that since I was too distraught. While I’ve been friends with G for a while this summer we spent damn near every day together since I was sad.

Prior to the break up H never mentioned being insecure or jealous of G but when we decided to get back together in October we agreed to be more open. He expressed to me that he is upset by the relationship and the closeness. He is afraid that G is going to swoop in and steal me.

My issue with this is that I’ve been friends with G since we were 5. G is considered family in my house and in his I’m another sister/ daughter. I have absolutely no idea what to do about this. I feel like I can never reassure H that G and I are just friends and neither of us are romantically attracted to the other. It’s a little overwhelming.