As someone that’s enjoyed this for years, I hope I can provide some expert advice here!
Firstly, I assume your man is up for it? Don’t just go probing around there without some sort of willingness and expectation on his part. Despite progress of sexual attitudes, there is still a taboo about a man enjoying bum action and this can create insecurities.
Assuming you’re good to go on the above, then the prostate is about 6-8cm on the upper side (towards belly button). Whether or not you can reach it easily may depend on the length of your fingers. There’s a toy on the market called the rude-boy, which is perfectly shaped to hit the spot, so if you can get your hands on one of these then that should really help give a good idea of where you’re aiming. When you do hit the spot, then gentle rubbing is the order of the day. Obviously, gauge his reaction as to what works for him.
Having said all of that, don’t get overly obsessed about the prostate when playing with his bum. The whole area is super sensitive for men when turned on. Just having a lubed finger or 2 thrusting in and out can feel amazing, and if he’s feeling up for it and comfortable about being on the receiving end of penetration then move on to a dildo. This can feel great if playing with his penis at the same time, but it also just feels great on its own. I’ve personally had the most powerful orgasms I’ve ever experienced from just anal penetration over the years, and its definitely more than just prostate stimulation causing them.
Happy to go into more detail on the mechanics of the above if wanted, but am wary that it may cross the TMI boundaries, so let me know. Either way, your partner is a lucky guy for having someone that wants to pleasure him in such a way. Enjoy!