He should also try:

1) biofeedback
2) metaphor therapy
3) mindfulness - unlike the paradoxical approach with which he would POSITIVE AND ACTIVELY engage in thinking through the unwanted thoughts, Mindfulness would let him NEUTRALLY experience the flow of thoughts without trying to influence them in any way. Mindfulness is extremely popular and pretty much everyone and their brother has been trained in it, so the fact that the counselors have not suggested it is unfathomable
4) journaling - writing down the thoughts and dreams ASAP, in detail

There are other ideas, but try these first.

And no, you did not make any MISTAKE in telling him about the one night stands. Most people would not have reacted the way he did, so you could not predicted his reaction, and your only mistake was that you were not PRESCIENT, but this is not a mistake - it is a shortcoming that many people share with you.

It would have been a mistake to tell him that you had been habitually slitting the throats of innocent infants and then stopped and decided to build a quiet happy life with the bf. After that kind of disclosure, your bf could have been reasonable expected to be haunted by bad dreams.