Perhaps because he desires and loves you so much that he can't think straight. Or, that he is afraid of losing you, and is worrying about whether his actions are correct, or appropriate. Or, he is in a situation which is overwhelming him, making him worry about it all the time. Try hugging him, holding him, talking to him, reassuring him. Don't try to take him to bed, because he might feel pressured to the point where he could not perform.