This has always been a tough question. You can try hinting at it jokingly, or saying you accidentally stumbled across a website. But you gotta realize that if he is into it, he’s going to be just as cautious as you. He will most likely pretend to be disgusted by it or brush it off.
Heres my situation. Ive always been attracted to amimals. For as long as I can remember. But I was also attracted to humans. I got into my first real relationship a few months ago and quickly realizedI’m really not attracted to humans at all. But I had a good suspicion she was zoo. She was fun to be around and do things with but I liked more as a sister that a girlfriend. Well, she was also kinda of a sex addict. I hid my innattraction to her by telling her I wanted to take it slow. Which worked a little. But she was always making jokes about sex and surprisingly often about send with my dog. Our first “date” I took for dirt bike riding and we brought my dog, my male shepherd. At one point they tripped over each other and his balls kinda went in her mouth. Most girls would gag or make a big scene but she just kinda took it. Joked about it kept going. She probably mentioned it 15 more times that night. From then on she made jokes about my dog being her boyfriend and loving her more than I did. Not a day went by where she didn’t mention his balls in her mouth. Over the next 2 months of us dating she made frequent references to sex, being zoo, bdsm, and I came to the conclusion she was into a lot of kinky shit. Even though I’m very zoo, I always pretend I wasn’t. It’s really hard to tell with things like this. The risk it took great. There was one time she straight up randomly said “So I found out you watch animal porn.” I kinda acted a like nonchalant like it wasn’t true and said “how’d you find that out?@ At this point my brain was racing. Of course I watch animal porn. But I’m always very careful. She doesn’t have access to my phone. I don’t have a PC. The only way was I have her my only PlayStation. But I never watched porn on it. In addition to figuring out how/if she knew, I was trying to figure out if she was into it. She said “well I joked about it earlier and you didn’t respond.” If I’m focused on something I often phase everything out, which is what must’ve happened. From then on I couldn’t figure out if she was joking, if she really knew. If she was into it and trying to do what you’re doing and hinting a little. I never did work up the courage to ask her outright before we broke up. Who knows, maybe things would be different.
My advice to you, really get to know your bf. Love him and develop a strong relationship with him. Then one night, start talking to him about sex. Say something like hey I wanna get a little kinkier. Start slow. Something more acceptable. Gauge his reaction. Then move on to your dog. Try to make it sound more pleasurable for him than you. Maybe something like “I want you to watch me.” Start kissing your dog a little. Or maybe start jerking him off a little bit. The trick is to get your bf in the mood first, and make it seem more for him that you. Go from there. Once he gets his foot in it’s smooth sailing. Obviously everyone’s different, and you know your bf better than me.