Advice please?

I’ve been seeing someone (G) for a few months and we are in love [yes, I know it’s a short amount of time but my husband and I fell in love within a few dates and we’ve been together now for 4 years so it’s not impossible].

Unfortunately, G and his other partner (S) are monogamous and will be becoming exclusive soon (timeline TBD, though I did request something more concrete last night if he can get that from conversations with her).

We have always been open and honest with each other. G knew I was polyamorous and thought he would be able to do it too, but he knows now he can’t do it longterm. He just doesn’t have the emotional bandwidth, and I respect that.

I thought we’d have a few years between this and G deciding to go on the path to marriage/kids with someone (either S or someone else)… but we don’t. G and S talked recently and, without knowing the details, they will be closing up “soon”. Basically we have a month or two now.

But G and I had agreed from day 1 that we’d stay in each other’s lives. So now it’s time to think about platonic friendship. I’ve transitioned to friendship from romantic relationships before - with those I hadn’t had much sexual attraction to but whom I loved dearly. This is… way different.

How can we pull off this transition in the healthiest way possible once the time is up/starting now?

TLDR: how do you healthily transition from full on sexy romantic coupling to platonic friendship with a partner?