R.D.
I myself would think that he may have some infection in his urine. I wouldn't worry yourself sick over it. If it was something very serious they would have had you take him in to the hospital. If nothing else ask your son if when he goes to the bathroom does it sometimes sting or hurt? He will answer you. I think it is nothing more than a bladder infection that antibiotics will cure.
Don't worry mom, everything will be fine. We all go overboard when we get that call and then think of how we reacted after going back in.
Lots of luck mom, and try and sleep well. Everything will be fine!!