It is quite normal that overexcitement can result in losing the erection quickly. It could be worth your while to read up a bit on premature ejaculation - where one of the complaints is that as soon as he penetrates he loses his erection. That could shed some light on understanding it and how to deal with that. The fact that he managed a successful intercourse afterwards should indicate that you two are capable of a successful sex life. Kegel exercises could help him to gain more control over his erection (PC muscles). Men with similar concerns often masturbate beforehand to minimize the over sensitivity and overexcitement.
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