My son had salmonella poisoning 2yrs ago, and to this day he is having bowel prob. Is this normal? He is 8?

He cant hardly go to the bathroom at all for a wk or so at a time. The doctors have put him on a stool softner which does not help him. When he finally does go it is still hard as a rock and really dark. Please help, im worried that something mite be wrong with his bowels or rectum. He is my little boy. Im worried.