This topic comes up quite regularly, there’s no easy answer or 100% safe, works all the time kind of answer.
Everyone behaves\reacts differently. The fact that you’re both extremely into the BDSM lifestyle is already a positive.
You’re both out of the normal “construct”doing things that are normally considered filth and unacceptable by social/religious construct.
Ultimately that’s a good thing, it means you were able to break down and act on some kinks and fetishes.
This is where my advice comes in, I would say gradually talk about other fetishes/fantasies, even if you’ve done so already maybe those need to be talked about in more detail. And I’m not talking about this lifestyle, this is the last one to bring in once you are 100% confident it will work out.
I’m talking about lifestyles like cuckolding, Hotwife, swinging. If you end up finding out that he does in fact, fantasies about you being with someone else, that’s already a small step in the right direction.
Just the idea, that he’d want to see you with someone else already brings that structure on the table, even if he gets involved, that’s all good, even better I would say.
With that at play, you could start role playing this fantasy.
Start role playing the idea of you being with other men, women, ideally you let him tell you who he fancies seeing with you and you role play it.
With time, it gets a little stale so you start making it a little more spicy, if both your imaginations can run a little wild start bringing Aliens to the mix, mystical beasts, things of that nature, introduce things that you are both familiar with. Being descriptive is recommended, helps selling the fantasy and desire, then with time he may bring in real animals into the mix, that would be where you capture the moment.
Roleplaying is a way to break the ice sometimes, don’t get me wrong, sometimes things don’t go past role play.
But if you have the right partner, it can work.
I really fantasised/desired a Hotwife, we role played it for a while and now I actually have a Hotwife. We’re both loving it and she’s into it.
Finally, my last advice for you is, to be careful about his relationship with his dogs. How does he interact with them? Is he very protective of them? How does he treat them?
When I came out to my SO, even though it turned out she was also into this lifestyle, one of her first questions to me was, have you done anything to our dogs (though they were more hers than ours). But the way she asked was in a protective manner, aka if anything were to happen with them it would need to go through her first.
She clearly knows our dogs better than I do, she spends more time with them than I do. So she definitely doesn’t want anything “bad” to happen to them. I don’t hold it against her, she’s protective about them and rightly so.
So please be mindful of his feelings/relationship with his dogs.
He may not want anything of that nature happen to “his” dogs, however that may not dissuade him from looking at other owners.
Either way, stay safe, stay vigilant and please take your time with this, slow positive results = Long Term Gains!!