This could be a muscular problem - has he done any strenuous lifting or twisting and picking up anything?
The other problem in that area that it could be is related to the kidney, so it could be a kidney infection or kidney stones. He would have other symptoms with an infection like an urgent need to urinate or frequent urges to urinate. He could also have a fever, cloudy, dark or very strong smelling urine.
With kidney stones there is not always pain if they do not move and a stone can be as small as a grain of sand and can sometimes be felt like colic.
Make him aware that he should drink plenty of water.
A doctor would test a urine sample to see if there is an infection and would prescribe an antibiotic if required. There is no medication for kidney stones and they will come out eventually if they are small enough.
Get him to make an appointment to see his doctor so that he can get examined and given the correct diagnosis and treatment if necessary.
Could be a trapped nerve - I had something similar a little while ago in that when i lay down i would feel a stabbing/spasm in a similar area. I went for a sports massage which seemed to help
I don't mean to be cruel, but if he adamantly refused to see a doctor, there is not much you can do. Men are like that sometimes; but waiting could be the worst thing he can do. It could be an infection, tumor, or a myriad of other things. Since it is only when he lays down, it apparently is causing pressure on something. If he ignores calling the doctor, remind him he might find himself in the ER with a ruptured something and it will be a lot worse. Quick detection is the way to go.
Good luck with the push to the doctor!! And if he doesn't have a doctor, get him to go to Urgent Care, not the ER; they are busy with fractures, accidents, poisonings, people bleeding all over and deadly threats. This is not that, so far...