Well first off, guys like what they like when it comes to sex. They know very well what turns them on and what doesn't turn them on. They know what they want to try and what they don't want to try. Even if they keep it from others. If he has even the slightest hint of a desire for bondage in him, you would have picked up on something by now. Bondage is dominance and submission. If he thinks it's sexy, he would have possibly made jokes about spanking or he always wants to be in a position where he feels in control. A lot of guys use a sexual joke in a relationship to test the waters to see what you think is acceptable and what isn't. If you are sexually active and you never heard even an incline of a desire to dominant, then it probably isn't his thing.
And realize too that even if he likes the idea of something like tying you up, he may be so freaked out by the idea of actually making you a "slave". Bondage people want to be with other bondage people so if you too aren't doing that, I get the impression he isn't into it on that level.
But that doesn't mean he won't do it for you. But you don't show up one day with the collar on. I would joke around about very innocent stuff like spanking. If he thinks it's sexy, then go from there. But ask for things like being tied up with a silk scarf and see how that goes. Don't break out a dog chain and wonder why he is freaked out.
And also, FYI, getting into that stuff does give the relationship outside the bedroom a different dynamic. Bondage is never just a bedroom thing. It shifts very easily outside the bedroom and you will find yourself dealing with issues that happened outside of the bedroom, in the bedroom. The submissive person starts acting oddly more submissive in day to day things and same with the dominate person. And as sexy as that can sound to a person who loves bondage, it can get very muddy so if you have never had that experience, it's something you need to realize. Cause there are a lot of "WTF just happned?" moments when you start dabbling in that stuff.