My Husband Has Frequent Loose Bowel Movements And Has Lost

My Husband has frequent loose bowel movements and has lost about 15-20 lbs in about 3 mos. He recently had a colonoscopy which came out fine. He does not have any pain and is able to work each day w/o much problem and his appetite is fine. What diagnostic tests or scans, do you feel, should be done next to try to find what his problem might be. He is Caucasian, 65 y/o, no other known health problems. Thank you, Kathleen