Probably the most common source of pain is peritonitis. I assume that the clinic has checked his dialysate for peritonitis. Sometimes people have sensitivity to certain kinds of PD solutions and that can cause pain. Sometimes the catheter can move around causing pain.
How severe is the pain on a zero to 10 scale? Where is the pain? Is it constant or intermittent? Does he have it right after doing activity or some time later -- how much later? Does your husband have a chronic cough? Does he strain anytime? Does he jump? These are all things that can cause pressure in the abdomen. Here's a portion of an interesting abstract from 1986 on research with PD patients:
"The pressures with weight lifting were proportional to the magnitude of the weight lifted up to 50 lbs, but were lower than those during coughing and straining. The pressures were generally higher with greater intraabdominal fluid volumes, especially with jumping and coughing. Exercycling was associated with lower intraabdominal pressure than was jogging, and the pressures were only minimally influenced by intraperitoneal fluid volumes." Twardowski ZJ, Khanna R, Nolph KD, Scalamogna A, Metzler MH, Schneider TW, Prowant BF, Ryan LP. Intraabdominal pressures during natural activities in patients treated with continuous ambulatory peritoneal dialysis. Nephron. 1986;44(2):129-35.
A 2005 study in healthy people said that the greatest intraabdominal pressure occurred when coughing and jumping. It warned that heavier people with chronic cough were high risk for hernia following surgery. Cobb WS, Burns JM, Kercher KW, Matthews BD, James Norton H, Todd Heniford B. Normal Intraabdominal Pressure in Healthy Adults. J Surg Res. 2005 Sep 1.