Yes that Royal Canin is an RX food. If he is used to dog food, then I would get him the Royal Canin LS 14 and keep him on that. It is made for this type of issue.
Home cooking is tricky. You would need to contact a Pet Nutritionist to get it balanced and calculated for his weight/issues/active levels, and being that he is shooting additional shunts, I wouldn't go that route. I would stick to the Royal Canin LS 14 (dry) and some L/D (wet). And that's it!
Treats can be romaine lettuce, watermelon, apple bits (no skin)
He needs to be on supplements to aid his Liver and Lactulose.
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DogLiverDisease : Dog Liver Disease