"I have a 19-year old nephew who claims he's never had an erection not even a typical morning boner. Is this possible? What mechanism actually traps blood in the dick to make it and keep it hard?"
That would be unusual. But it is possible. Can he at least get semi-hard?
"The blood creates pressure in the corpora cavernosa, making the penis expand and creating an erection. The tunica albuginea (the membrane surrounding the corpora cavernosa), helps to trap the blood in the corpora cavernosa, sustaining the erection."
Additionally, the Michigan Institute of Urology, in their article, Vasculogenic Erectile Dysfunction, gets more detailed in what does or doesn't happen in the getting of and maintaining an erection. There are many forms and causes of ED; Psychological and physiological, And different ways of treating it.
Is it possible that nerve centers were somehow damaged? Is it possible that one (or more) of the interior sheaths (Tunica, or Deep Bucks Fascia) didn't form properly? Most likely, only a Ultrasonography could discover that. I'd suggest a trip to a top-rated urologist for a full-on genital examination.
Because, in my opinion, an otherwise healthy male his age should be experiencing multiple erections every day and every night.
