I have 2 poop eaters currently. I had 2 others, but they've gone to the bridge.
I've tried, pineapple juice, meat tenderizer (more than one brand) and the pills and even hot hot hot sauce. I tried them for prolonged periods of time. They did not work for any of my dogs.
We use a kennel muzzle with a stool cup. No, that does not prevent them from rubbing the muzzle into the poop, but very little gets into the stool cup. I hear that duct tape works well too. The only way to prevent that is to be there whenever they are outside. Poop must be picked up almost as soon as they leave it on the ground. I do poop patrol 2x a day now, that's down from about 5x a day when I had more dogs.
If this is a foster, and you have such negative feelings towards him, sounds like maybe it's time for him to go to a different foster home. That happens, sometimes certain dogs just don't jive with a family or a foster parent. Best maybe that he go somewhere where he is not alone all day.
And for whatever it's worth, my dogs don't have to be bored to eat poop, it's just that some dogs love to eat it, doesn't matter what you do, they love the taste evidently, at least that's what every vet and trainer I've talked to said about it. I accept it. I have had the poop puke in the house, it's so lovely!!! BLECH! But I love my dogs and it is what it is, nothing I can do to change it, I can just prevent it the best I can.
Looking at the picture of his muzzle, I can see why he's eating it. He can get it right in those big holes. Get a kennel muzzle with a stool guard, they work pretty well, much better than the one you are using I could imagine. Good luck.