Have you changed his grain or taken him off the grain? Maybe the senior grain has too much sugar in it for him? Have you tried putting him on a low starch grain?
Have you tried giving him oats? This works as a sandclear as well, but you can give it to them all the time.
as for the pooping in the corner thing, I have no idea how to fix that. My friends horse does this too and only in the stall!
"Take the time it takes, so that it takes less time."
"Expect a lot, accept a little, reward often."