To strap it on or not to Strap on...

My partner and I are in a monogamist heterosexual relationship, but she does me with a strap on occasionally. I think I was the one that brought it up at first, so maybe the situation is a little different. After you have had some experience pleasuring him anally, maybe strike up a conversation about which types of sensations he enjoys and different ways you could deliver those sensations to him.
There are lots of decent, basic strap-ons available and I would suggest not spending too much on one until you have been able to determine whether or not it is going to be used much.

I would purchase a cheap, durable harness first to see if you both enjoy it. There's a harness by Sportsheets that is really great, though a bit uncomfortable. If you guys like it I would recommend the Spareparts Joque harness. It's very, very comfortable.

There should be plenty of reviews and such for you to browse to determine what will fill your needs best.