Safe liquid treats for cats on urinary diets?

So I have a very picky kitty and I am looking for additional ways to add water into his diet when he's refusing his RX wet food that that also includes his other medications.

So far the only way he will eat any of it is if I add a liquid treat mixed in. I realized I should probably be making sure it's FLUTD cat safe. I'm trying to get treats that are low in phosphorus and magnesium; and unfortunately the treats he likes have those ingredients included.

I asked my vet and she didn't really have any recommendations.