from the symptoms you have described it would seem possible that your boyfriend either has the beginnings of arthritis in his back or the onset of sciatica.
i have suffered with arthritis in exactly the same position as your boyfriends pain for several years, and i am only 31 years old! you dont have to be old to get arthritis unfortunately.
at first, my doctor thought it was just sciatica because of the location of the pain and because of my age, but after an x-ray they discovered that it was the arthritis ridden bone pressing against the sciatic nerve, and this is why the pain was so similar to that of sciatica.
my back crunches and clicks sometimes, which is a symptom of arthritis.
of course, your boyfriend might just have slipped a disc or pulled a muscle or something else entirely, it is hard to diagnose back pain without more in depth examinations, so i would certainly suggest that he visits the doctor again if the pain doesnt go away or get at least slightly better in a few days.
sometimes you have to push doctors into sending you for an x-ray, so dont let him be fobbed off again!
i hope this helps