I also had another thought as to narrowing down what is making Logi suffer with his tummy.

when we had similar problems, we had to starve Siegfried for 24 hours, to allow him to clean his bowels out of all the upset, before gradually introducing the RC Sensitive diet. This worked brilliantly. We have had upset tummys after this, but usually when he had stolen banned foodstuffs (you name it, he wants it as long as it's human food!) and the RC diet has sorted him out within 24 hours. It might be worth trying a starvation period if he is continually suffering an upset tum.

Good luck.