when you lose sensitivity in your member position isn't really a point of cause, meaning that no matter what position you use if sensitivity is lost, then the erection can also be lost.

being erotic is a function of the mind, and can sometimes help with the maintaining of an erection. if one position is more erotic to your partner than another one (for instance cowgirl vs missionary) it may help to keep him in the game so to speak.

this is why I mentioned open honest communication. if you can know his deepest desires (almost everything outside of medication side effects can affect desire) then you are on the path to helping not only him, but yourself too.