I'd just like to add, as sobesmom said, simplify, simplify, simplify! I had a grey who came with pudding poop, several pounds underweight because of it. I did worm him, I think with panacur? (It has been 10 yrs) and that helped, but it took a few months of trying different things, one or two at a time, to figure out what he could eat and what he couldn't. I always had to be careful with anything new, but he lived happy and mostly healthy to 11 1/2. I also lost him to hemangiosarcoma Honestly, most greys aren't as food sensitive as he was, so hopefully you will find just a few things that are triggers for your boy. A probiotic and some digestive enzymes might help settle his gut while you are working this out. Good luck!
btw way, love "Lad the Inhaler"