As the name suggests, this is an anxiety about penetrating the female and attempts will result in the partial or total collapse of the erection. The erection will sometimes return fairly swiftly but will usually be accompanied by an attempt at rapid and too-early penetration 'before it goes down again' which results in more failure. If he can manage to penetrate with a partial erection, there will often be an eventual return to full engorgement. The problem can be sporadic or constant and usually responds extremely well to the correct style of therapy.
There is a possibility that the problem has a physical base (see pelvic steal and if this appears to be the case, then a medical consultation is essential.
This problem usually has more to do with a lack of confidence with females generally that it does about sex, even though it is when intercourse is attempted that the problem is evident. This should not be a mystery; after all, intercourse does not just require a male to be with a female but to be physically inside her of course. Since his lack of confidence is most frequently due to fear of rejection, it follows that this fear will be more profound when approaching the intimacy of intercourse.
The anxiety might be based on many things, including embarrassment, guilt or shame, any or all of which may emanate from a variety of causes. Self help is difficult here but our psychosexual therapists are available to help you.