research the benefits and just bring it up?

Can't help you on this one, my hubby won't even let my finger anywhere near his hole!

Start with a well lubed - well trimmed finger. It's free - I'd suggest trying while giving him a BJ. Do a lot of teasing first, the press lightly with the pad of your finger several times before trying to go deeper.

At each stage go slowly, pause frequently and reapply lube often.

Good luck - I had to more or less beg my wife to do this for me. She's use to it now and her technique is improving. Your interest should make it easy and pleasurable for him.

Others have great suggestions. I will add this. Get a toy that is useful for both of you in this manner. Something vibrating. Give him a slow teasing BJ, massage (maybe blindfold him or tie his hands if you are both a little kinky). You could also get on your knees and suck him in the shower and use soap to lubricate and "wash" his as and slowly work your finger inside.

Back to my main idea:
When you suck him use the vibrating toy on his cock/balls/perineum. Gradually work it down to his anus and let the toy tickle it...get it wet discreetly with saliva or lube after a while...use gentle pressure on his anus without penetrating....watch for his body language...does he spread his legs? lift his ass? Use your judgement...this is very pleasurable without penetration...if you sense he might be ok penetrate him slowly...if not, wait it out and try it the next time. Ask him occasionally how it feels. If you have open communication, suggest in one of your wickedest voices that "maybe we should try this toy on your Prostate like you do on my G spot?"

Your average man has a fear of being perceived as as being anything less than a masculine tough straight guy...so it is important that he not feel judged or that enjoying anal play is not equated with him being labelled as anything but straight. He may also be concerned with you perceiving him as less than "normal" if engaging in this. I think this is one of the biggest obstacles men face doing this for men in a relationship....of course some men/women just flat out don't like the idea of anything in their ass too. I felt that way at one time too.

Of course you could just tell him you read how mindblowing an orgasm a guy can have with a P spot massage too and how much it would turn you on to do it to him...that might work.