He could be sleeping with you regularly and be cheating. The fact that he is not sleeping may seem to indicate that he is cheating; but, there are other causes.
He may be suffering from some form of erectile dysfunction and be too ashamed to tell you. Some men would rather let you think they are cheating than admit they "can't get it up".
How is the rest of your relationship? If he is still as loving towards you as before, there is probably something else at play other than cheating.
Assure him of your love and try to talk things out with him.