Hi, I am also a newbie to this, I got into it for prostate health.
You man will need to evacuate himeself well even cleaning himself out with an enima and or bidet.
I prefer a bidet because I use it daily because I love to know I am clean and not smelly downthere at anytime.
After he is purged and clean then it should be safe to proceed.
He may want you to use your finger but I suggest using toys designed for this at first, obviously you will want a few toys one smaller one to help loosen him up and get him good and lubed up ( they even make a lube injector )
I highly recommend getting a prostate massager that massages both the internal and external zones, they have these toys where they just vibrate with different intensites and pulses, I love pulses more than just vibration.
As a added bonus with the massager inside him you will also feel the pulses and or vibrations right through his penis. ( my OH loves it )
From what I have learned so far, he must be relaxed and it helps if you get him excited with oral at first this helps to make the prostate inlarged slightley making it easier to reach and more pleasurable once you do reach it either with a toy or your finger or fingers.
Until he is streched out some I suggest having him insert these toys on his own and have him tell you how to do it after he has had a few solo sessions.
He should lay on his side with his knees close to his chest but in a comfortable position then slowly work the toy in, at first it is uncomfortable because of a the narrow opening but with enough lube and time it will eventually get past the tight spot and slide in.
At this point he should relax gather himself and get used to the feeling before turning it on.
He could get back into solo play or you could join him if he is comfortable enough.
Doing this for the first time can be embarasing at first but if you assure him you are also into this and support him though this he will become more confident about anal play.
Lets say he has had a few solo sessions and is used to the toy and is comfortable with sharing the experiance with you be prepared for him to cum fast and hard if your giving him oral while the toy is on so if you want sex besure not to take him too far or use a cock ring to extend this session and make it even better.
I know I love it when my OH manipulates the masager gentley pushing it in further wiggling it around and playing with the speeds and functions while giving oral.
Because I am so new to this and I am very tight down there pulling the toy in and out can be a turn off if it is too large to move in and out of him, a regular shaped yet small prostate toy or your fingers would be better choice to move in and out of him.
I have yet to have my wife insert her finger because she likes her nails and I will not allow any sharp objects down there he shouldnt either.
if your willing to trim your nails and you have your OH onhis back with legs in the air you can insert your finger then two fingers until you feel his prostate which I am told has a weird texture
While your in him move your fingers in a come hither motion along the prostate or slighlty tap it in time with oral if possible it may not seem to do anything at first pleasure wise for him yet it should not take long before he is in heaven and looking at you like your a sex goddess that is when he recovers from his most intense orgasm ever.
Again I am new to this and I am also into learning more, perhaps you can come back and letus know this went.