I’m sorry you and bun are facing this.
Were the bladder stone confirmed with an X-ray?
I remember BinkyBunny’s rabbit Jack had a bladder stone. From memory he had it removed and she modified diet to prevent it reoccurring.
One thing used was Polycitra which is a potassium citrate solution. You could ask your vet whether something like that would help, regardless of whether your bunny has surgery or not.
Stones vary in what they’re made up of so Polycitra may or may not help. My thinking is it wouldn’t hurt to try but I do not know if there are side effects. Best ask the vet on that.
On a side note, GI Stasis can come on as a result of pain. Is your rabbit currently on pain medication?
Not opting for surgery has the potential of be becoming equally expensive, especially if stasis recurs and hospitalization is needed. Please be aware of this. I absolutely do understand your reluctance to put him through surgery though.
I agree about looking into finance options. It would help for any health concerns with your rabbit in the future also.