My wife hates anal on herself, but gets very turned on by putting a butt plug in me, and I find it pretty hot. Just start with a finger gently stimulating around the anus (before you ever go in). My wife does this and it drives me nuts. Eventually, you can use your tongue (my wife won't do this for me. Also, I would love for her to want it on herself, but she has not interest in her own back door). Tongue his anus while you stroke him and he'll go nuts. Then start putting a finger in, very well lubed with saliva. Advance to a butt plug when you're ready. A dildo or butt plug jiggled while you give a blowjob will send him into orbit.
The key to relaxation of the sphincter is to push out as if having a BM. That's how it relaxes to do its job.
