Sounds like you've been through quite a lot, grown some, and much of the trust that was lost a year ago has been rebuilt. I hope that's the case, as it'll make what you're proposing easier (maybe even just possible).
I agree with PolyGamerGirl that purely sexual relationships are somewhat harder to maintain in real life than in our imaginations. Our biochemistry fights us here every step of the way. Sex (and orgasm) are literally designed to bond us closer together.
Your husband's history also points to a need for other relationships, not just other sexual partners. It is important for him to reflect on his life's experience to determine how strong that need for emotional connection actually is (difficult for some of us males). I know that this was key to my realization that I was poly. I reflected on my history and found that it wasn't random sexual dalliances that were satisfactory in the least. Instead, I found that I emphasized longer term connections. If he's similar, the causal restriction will fail on one of two counts.. (1) he'll be miserable or (2) he'll inadvertently make strong emotional bonds without being to stop it. Maybe both.
Accepting him fully would include accepting that aspect of his needs as well. A relationship of mine that recently ended was impaled on this very issue. She had much less struggle with my sexual connections. But, couldn't integrate the idea that I might becoming emotionally connected to another.
It is difficult to manage keeping things casual. Perhaps swapping out partners before emotions begin to develop. But, that presents other risks. Also, emotional connections develop at different speeds with different people. Neither you nor he can accurately predict ahead of time when that might occur. Breaking things off after that emotional connection has developed is a recipe for bringing more pain in your life than necessary.
I wish you well in navigating through this.
Male, Straight, Poly
“Instead of getting better and better at avoiding, learn to accept the present moment as if you had invited it. And work with it instead of against it. And making it your ally rather than your enemy.”