Liver can cause runny stools. Humphrey's fed raw (DIY) and as they're supposed to have a certain amount of offal , including liver (can't remember the exact ratio now but can look it up if you need it), I used to give him his weekly ration in one meal but it always made him runny, so I now spread it out over the week. If you're giving him larger amounts as treats, then maybe its a good idea to cut it back a bit in his diet - or replace it altogether. In the raw food book I have they suggest giving the bone ration with the offal ration as bone tends to have the opposite effect, so the one helps counteract the other. Hope that helps!