I would do away with the biscuits as they break down in the gut far slower meaning his digestive system is constantly working. Also try adding beg to his diet I use the frozen mixed veg bags from Tesco their cheap and spot on for the dogs. Adding more bone to his meal will tighten up his stools. You could also use some sf50 powder sprinkled on each meal. The thing is with digestive problems is that they often take a fair old while to settle so anything new you try to get a good result needs to be done for at least a month to allow the dogs gut to get used to it ........