He may have some performance anxiety. You may take intercourse off the menu for a few weeks. Then when he has an erection you come on top and do all the work while he just lies there and does nothing.
He may also benefit from taking testosterone replacement therapy.
Ask your partner to do slow,deep and even(inhalation and ehalations are eqial). Google it, learn it practice it for 10 minutes twice a day. while doing inter course, slow breathing will help hem to keep erection for a longer time. Practice for a month to get full advantage.
In fact he has to do something. The first thing he has to do is to quit porno. Because of porno, he is not aroused by you as he dantasizing an actress type of lady. He can do male deer exercise, kegel exercise. He may search breathing exercises. Google these exercises.For female there is a good technique which will be found in 'breathe to orgasm' Google this phrase and find Anniesprikle org. It is an interesting get orgasm. It is a treatment as well as pleasure tool. You will have to learn with some efforts. For man another exercise is microcosmic orbit. Ask your partner to do these exercises.