I would give it at least 2 weeks, preferably 4 weeks on one type of food. Also keep in mind if his system is sensative than feed him good quality treats as well and only 1 kind, but if it were me I would not give him any treats until the problem is under control, it may make it easier to find out exactly what is causing his stools to be loose.
You say his stools are loose sometimes that may mean he can not hold it as well as a formed stool. Has he had a vet exam for this problem? He may have something wrong w/ him.
Feedin him at the exact same time should help some too!

Maybe when he is outside he eats the other dogs poop & not his?