First of all, nobody knows your mind but you. You are the only expert on your own personal psychology, and so my suggestions should not be taken as informed since I in fact have zero information on who Sam is as a soul and how his particular consciousness operates.
That being said...
Somebody may have already suggested this (there were a lot of long posts that I skimmed through or skipped altogether), but has Sam thought of smoking DMT again in order to better come to terms with what he experienced? Indeed his understanding of what he experienced is mere metaphor oatmeal by now, and so perhaps he can find reconciliation and healing on the plane of existence from which his anxiety originated.
I know that everybody else has recommended that Sam stop drug use altogether and find other recreational distractions, but I think it would be difficult for Sam to find recreational activities intense enough to overshadow his trauma outside of the realm of psychedelic drugs. I recommend something pleasant like MDMA or LSD. (What I am about to say assumes that Sam has little experience with other psychedelics) I think that if Sam continues his experimentation with psychedelics, he will find that his disturbing bedtime experiences are merely a result of his subconscious fascination with an incredible first experience. As with sex, the first time with psychedelics can be filled with anxiety and can even be haunting, as in Sam's case. It's not until one has experimented more with sex or psychedelics that one better understands the experience and, through it, one's self. With such understanding (which primarily comes from additional experience) one becomes wiser and more resilient, and sexual or psychedelic experiences become more enjoyable as a result.
In other words, Sam should relax. He's making too big of a deal out of a DMT experience. I know it seems impossible, but Sam should try to downplay what he experienced as much as possible. He can undermine some of the trauma of the experience by trying to take the experience less seriously. One way to do this is to use DMT again, and again,... What Sam will find is that the trauma of his last experience is replaced by the trauma of a new experience. Eventually one learns what to expect from DMT and then the inherent trauma of the DMT experience is no longer traumatizing.