Diarreah is a pretty serious problem. I'd probably have the vet out. There are a couple things I would look at though, one is what type of soil do you have? Are you feeding him outside? If he's eathing outside and then spilling his food on the ground, then picking it up, he could be suffering from sand in his belly. You could attempt a round of sandclear to see if it would help. I don't think it would hurt.
The other thing you could try is mixing some yogurt into his feed to see if he's got some bacteria in his system. But I would probably go ahead and make a vet call...
"Be a best friend, tell the truth, and overuse I love you
Go to work, do your best, don't outsmart your common sense
Never let your prayin knees get lazy
And love like crazy"