Coming from the perspective of someone that wants to open up, I fully support your approach.
I understand what a big decision it is and how much it is asking of my monogamous spouse. If you're taking an active role in trying to set what is acceptable and what it seems more than fair that he should reciprocate.
IIs there a chance he's already crossed some boundaries, and is feeling really guilty?
Me: 39 straight male in a V with
Wife: Kay - mono female - married 17 years
LDR girlfriend: Susan - poly female - 3 year relationship
Stakes - very intimate friend