Since they've been together all this time, it's not likely a territorial or behavior issue among them.
Since he didn't do this when regularly eating his 'old' food, it's probably not that (as you've discovered by trying to change foods).
You've returned to a favorite litter that he's used before - so it seems you've already anticipated and investigated all the possibilities that could be relatively easy to treat. I think you have to consider that this cat has an intestinal problem. It could be something simple to treat, once it's identified (he needs a vet).
If he gets a clean bill of health, the solution may be more tedious, but if he is found to have an infection (it could be bacterial, fungal or even a virus), chances are he's good as gold in a matter of days once given the proper meds.
Of course there's a possibility of age-related disease like diabetes or liver disease (but let's hope not).
(You need to keep a close watch on the others and if this one is found to have an infection, they may all need to be treated, depending on what kind it is).
More information is available here:
http://www.peteducation.com/article.cfm?cls=1&cat=1310&articleid=158
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You remain responsible, forever, for what you have tamed.
