I'm sorry to hear that Gizmo had a stone!! Poor little guy
Did you have the stone analyzed? This is really important in order to know what the proper diet will be for your baby...Struvite, calcium oxalate, ammonium urate...They each require different things/restrictions. Also, it's best to feed either canned food or add water to the dry food. Even if your baby drinks a TON of water, it's still not enough, particularly for stone producers. If you can keep the urine dilute enough, it will help prevent stone formation.
Good luck with your baby!!