It is perfectly normal for most men to be only semi-erect during a pegging session. The main factor is the blood flow that gets redirect to the anal area where most of the stimulation is taking place. Since a male erection is mostly create and maintain by that same blood flow the anal diversion is reducing the available pressure supplied to the penis.
There are ways to keep him hard or to bring him back to a nice erect state while you enjoy ramming his butt.
1 – The manual or oral stimulation
2 – The vibrating devices stimulation
3 – The cock ring
4 – Spanking
5 – Hardon cream
6 – Erection pills
Vulnerability is part of the turn-on for men, but that does not necessarily mean it is easy for a man to go to that space or that it is comfortable for him to be seen there.
Being able to trust that the woman understands this role reversal and will not suddenly judge him when he softens and receives helps him to more fully relax and allow that vulnerability to happen.
A man can orgasm from combined cock stimulation and pegging, which men say is about 10 times as strong as an orgasm with just cock stimulation (and produces much more ejaculate). Some men can also orgasm just from pegging. A prostate orgasm for a man is sort of like a G-spot orgasm for a woman. And then there are men who can have multiple orgasms from prostate stimulation.