As a top who has fucked quite a few guys, I notice that not all bottoms will stay erect or cum when being fucked, or that his erection will come and go during sex. It doesn't usually worry me, as long as he is enjoying it.
I always let the bottom choose whatever position he prefers, and often we just change position as he likes. The pleasure he gets from it depends a lot on the position, how experienced he is, and the size and shape of the penis.
I have a large (above average) penis and sometimes guys are a little bit unsure about taking it, or are inexperienced. In this case I always suggest that we lie on our sides, bring his knees up a bit, use lots of lube and go in slowly and gradually. In this position he has lots of control and can put a hand back on my thigh just to indicate if he wants me to slow down, or he can just pull forward himself. I never try to rush this stage no matter how horny I might be.
It is also important that he needs to be in the right mood, and relaxed, not worrying about possible pain etc.
Eventually, hopefully we'll get to the stage where my cock is fully inserted - or as far as he finds comfortable, and I will keep still in that position for a while. Either he, or I will start gently thrusting, gradually increasing speed and length. If he's not saying much to indicate his pleasure, I will occasionally ask him if it's okay, or if he wants to try a different position. Usually it will now feel pleasurable for him, once he's used to it. It's normal for it to feel a bit uncomfortable when it first goes in, but once there, it should feel mostly pleasurable.
I've had a few times where we've tried and he just finds it too uncomfortable or painful and we have to stop and do other things. It's not all about anal - plenty of other things to do.
One guy I tried fucking - he really wanted me inside him, and we tried a few times and he couldn't manage it. We did different things for a while, covered each other in lube and did horny slippery massage and sliding about on each other, our hard cocks between us. Eventually he flipped over, and I'm sliding about on his back, kissing his neck and ears, rubbing my cock in his crack.. he's flat out, legs spread, so I just gently start teasing his hole with my cock, or slide it between his legs. After enough teasing and getting him relaxed, he was so horny, and said "Oh, fuck! I want you inside me!". Then I switched from rubbing my cock in his crack, to sliding it in with all the lube. He was so relaxed and horny, this time it went inside no problem. We had an amazing fuck!
One other thing you can try is to get a simple prostate massager. I've tried a few fancy vibrating ones etc but the one I find works best is just a simple plastic one. Sometimes he doesn't cum, or I will cum before him but he still wants to cum. That thing usually does the job as it's quite intense stimulation.
Just play about a bit, try not to take it too seriously, have fun trying different things and getting to know each other's bodies and what turns the other on.