When he had his last blood work done ~ are all his organs (liver, pancreas and kidneys esp.) are doing what they should be doing? And the results of fecal samples taken on a regular basis?
Does he take prescribed medication of any kind on a regular basis? Have you ever tried eliminating the supplements to see if the digestion improves?
Any of the others in your household (furry and skins) have any
you know what I mean) problems?