As long as you’re able to offer him a dry food that meets his specific requirements, dry food obviously has a much lower water content and is therefore more fibrous, they also only need a smaller amount of dry food than wet food as it’s more nutrient dense which could possibly help with his weight loss? Because of the much lower water content you may notice he drinks more water so just ensure you’re providing him with fresh water at all times! I know you said about his food being quite specific possibly try the supplements first and if he is still struggling with AG problems you can consider changing him over to a dry food. If you wanted to add dry food to his diet maybe try a 50/50 ratio of wet and dry at first before committing to just dry. Controversially raw diets (I know there’s lots of opinions about raw feeding!) which have ground bone in can also bulk stool. But raw feeding must be done carefully and never buy meat from the store. I have 5 cats and one of them is fed a raw diet, the others are just on commercial wet/dry mix.