I agree with your cautious side to take it slowly. Sometimes our exuberance can cause us to scare someone off, and the slow approach is appropriate here. May I explain?
I am and always have been straight, but I've known that I was addicted to enemas since the age of 12. When I was 27, I met an MD (Board-certified gynecologist and gastroeneterologist) as her flight instructor and as it turned out, we were both attending the university in town. Being an excellent teacher, I lit her cerebral fuse and shortly afterwards, she reciprocated by opening an entire sexual world to me. She not only loved enemas but she was also very active anally, and the first time she ever touched my anus was a dream come true.
Years before, she'd asked one of her gay MD colleagues -- also a gastroenterologist -- what it was about anal sex that turned gay men on, and he actually allowed her to find out (in a hands-on clinical sense). Not long after the two of us met, she passed this knowledge on to me, using her fingers and a small vibrator, and it rocked my world!
In fact, when I woke up later that night with her in my arms, I was suddenly frightened by what I perceived to be a threat to my "manhood." I didn't know what to think: was she indirectly trying to tell me that she thought I was gay, or was I discovering that I was gay? I certainly couldn't vocalize my fears and this created, to be honest, a huge dent on my male ego.
I wouldn't see her; I wouldn't fly with her; I wouldn't meet her for our shared sandwich at 11:00 every school day to "share" her sandwich. I shut her so far out of my world, slamming the door on her so hard, that we nearly didn't survive this as a couple. To her credit as a woman and a doctor, she recognized what had happened and would not let go of me until we talked it out. She explained that what she did was an act of love -- certainly not an "accusation" or suggestion that I was gay or a test of my manhood. It took a lot of patience on her part and sensitivity to what could wreck the most fragile part of any man's ego.
It's not that I ever wondered if I was gay, fantasized about it or secretly worried about it. What she did took me by surprised and the fact that I liked what she did was what shook me so hard. Being a lot more mature now, I accept this and can understand it. But i relate this experience to tell you that it takes a great woman, a great man, and a lot of sensitivity to successfully introduce this into a relationship. It can and is done -- but you, dear teacher, have to realize that it can be introduced by using either a lot of finesse or brute force.
If everything else is going well for the two if you personally and sexually, may I suggest one method that you could employ to introduce rectal/anal play?
When you next take a shower together and you're both getting soaped up, lather the soap us in both hands, and standing to his side, your hips against his side, use one hand to play a little with his penis and testicles while the other hand sneaks around his butt. If he doesn't pull away or act as if this is uncomfortable for him, let your finger slowly drift its way up to his anus and with everything going well, use the finger to soap up his anus, slowly moving it around and around without penetration. Don't take it too far. Take it slowly. Time is on your side. Slow is smooth and smooth is fast.
I hope this helps.