Had the stone been confirmed via am x-ray or ultrasound. My friend has a rabbit whose vet thought he could feel a bladder stone but it turned out to be sludge. They were able to flush the bladder.
I think a member here had a similar experience with there rabbit.
Without knowing the composition of the stone, I don’t believe there is a way to reduce its size. I may be wrong!
In humans there is a procedure using shockwaves (lithotripsy) to break down size but I do not know if it’s used in veterinary medicine. One recent study were rabbits were used for testing (
) just resulted in damaged tissue and other complications. Although, it was not used on stones.
What is his diet and activity level like? Reducing his weight will be beneficial for his health over all. He then may be a candidate for surgery.
If he loses some weight and becomes more active, it should help prevent further stones developing.
It might be an idea for him to go on an antibiotic also. To prevent (or treat) bladder infection. If the environment was suitable for a stone to develop, it’s likely suitable to harbour pathological bacteria. A urine test may show if there is infection and give idea of the pH. Rabbits have more alkaline urine but overly alkaline urine is problematic as is acidic urine.
Have you noticed he strains to urinate or any other unusual toilet behaviours? Any retention of urine exacerbates the situation unfortunately.