I buy frozen green tripe chunks from the pet store. As already said, it's high in fat so puts weight on very easily. But it's also good for minerals etc. Whatever you do, don't cook it - the smell is disgusting & will cling to your house for days!
I always feed twice a day, right through the dog's life. Also, you often don't need to feed as much raw as you do commercial food. If he's burying food it's because he's had enough, but doesn't want to lose it. If he was leaving it because didn't like it, he'd just leave it alone.