For diet, get the diarrhea under control first.

Overcooked white rice and boiled chicken is a good bland diet to help stop diarrhea.

Leg quarters can be a problem for some dogs when they are just starting out. The darker meat and harder bones being the culprit.

I would also break it up into 4 small meals a day. Eventually, after he is tolerating the diet well, have at least 3 meals until you start going into the adult amounts of feeding.

You could probably go to 2 meals earlier, but I'm just being cautious since he is already having problems. Use your judgement on that once he is doing better.

Can you get chicken backs?

They are really soft, easily digestible bones and lean meat. You'll want to remove all the skin and visible fat at first. Once he has a few days of good poops with that, you can start leaving a small amount of fat on.

Then more of the same. After he shows to be doing well with the small amount of fat left on you can leave a little more, and so on and so on......

For right now, you don't need to worry so much with organ meats or meat variety in the diet. That will come in later after he has done well with the chicken backs.

Organ meat and other variety is added the same way, though. Small amounts and gradually building up to the desired amount.