Any tips on encouraging him to explore his own options?
At this time, your husband is not monogamous. He's already been with you and your other female lover and this is already Open to you dating women... so he knows you could date. It's not a Closed thing.
He is monoamorous -- he only wants to love one person, and that person is YOU right now. Don't push him to love others, or tell him to "look elsewhere" too much if he is indeed firmly monoamorous.
It could hurt to feel like his love gift to his spouse is not valued by his spouse. YKWIM?
Demonstrating with your behavior that you love him, value his love gift to you, build a base of "secure" there? That could help with this other question...
Any tips on warming my husband up to the possibility of me seeing other guys?
DADT -- like totally
DADT? Tell me ZERO nothing?
Or tell me THIS stuff, but don't TMI me. The stuff that matters to my health -- # of lovers, names, sex labs, etc.
There's a difference there -- if he's going all "ostrich" about it from fear/insecure, that's not going to be good.
If you wish to date other people and want to renegotiate the boundaries on that, could ask him what needs to happen first so THAT can happen with his "goodwill" and his "secure" on board.
It's good you are going slow and not pushing though.