Most likely he is suffering from performance anxiety. This usually means that the man feels responsible for satisfying his woman and he becomes so preoccupied with thinking about that, he goes out of his body into his head and loses his erection. You must let him know that you are responsible for your own pleasure and orgasmic response, not him. This does not mean he is irrelevant, but only that he can’t do it for you. He can’t make you come. He can’t make you orgasmic, only you, the female, can do that for yourself. Of course it helps if he is knowledgeable, skilled, and attentive as a lover, if he goes slowly, and if he genuinely cares for and loves you.

 
The other issue is making sex into a goal oriented practice, i.e., it’s all about getting to orgasm. Ironically, if you both drop the goal of getting to orgasm, you will have lots more of them. Instead, replace the goal of getting to orgasm with the purpose of joining in sublime ecstatic union. Focus on giving and receiving pleasure. Be fully present in each moment, paying complete attention to each touch, each breath, each contact with the eyes. Let orgasm take care of itself.