Honestly, with the amount of water he's taking in, there's no reason for his poop to be rock hard. Gravy, even wet food in general, doesn't sound like the problem.
His body simply isn't getting enough nutrients to produce a normal waste product. If he's overdrinking, he may just be trying to fill his near-starving tummy.
Changing a food suddenly isn't generally a good idea but get rid of the old kibble asap, and understand that it's going to cost more than planned to feed him properly.
I don't know what food you found but I'm going to do something that I don't normally and tell you to go buy a small size bag of Purina Pro Plan (no substitutions on the brand) but it can be any of the regular adult versions. Also buy several cans of the Purina Pro Plan regular adult food.
The dry food will come in a 6lb bag size so you can try it.
Dry Dog Food | Purina
Any of the full size canned food
Wet Dog Food | Purina
For the first couple of days mix only the new Purina PP wet food with the old Cesar and
don't stint! Feed to his weight! Follow the guidelines!
If he doesn't get a further upset tummy then fade out the Cesar and keep him on the PPP wet
but feed to his weight!
Next, in a couple more days, start putting the PPP kibble in his bowl but only about half the portion in the chart above. Top that with the PPP wet food, about half that portion.
Ideally he's getting fed twice a day, but remember that once you mix wet with dry, that food is only good for a hour or so if left uneaten. It can be refrigerated to try again later but that'll be hit and miss for success.
This requires more math as you'll need to split the amounts in the guidelines into two portions.
An alternative plan is to feed the PPP wet half portion as one meal and have the kibble available totally dry for him to graze on thru the day. This works only if there's no other animal in the house that might go for his left out food.
Try this for a couple of days. If he takes to this food and eats enough to support his weight, I'd expect normal stools sooner than later.
If he doesn't like the kibble, don't worry, the big box stores usually have a "take it back" policy, so check on that too.
(I should bring up that the amounts given in the feeding guidelines are given as if that is the only food given thru the day. If he's getting treats or snacks additionally thru the day then the total PPP fed can be reduced a bit. You'll need to have some idea of how many calories he's getting thru treats and snacks to adjust. For the first week of the experiment, tho, just feed per guidelines.)