Hi,
May I ask if the urine was analysed for sediment and the type of stones [oxalate, urate, struvite...]?
Are his uric acid levels normal? Has he had the urine cultured and tested for any infection.
Were the kidneys otherwise normal on ultrasound? no cysts?
Prevention requires an understanding of the uderlying factors pre-disposing to stone formation. Diet could be important depending on these factors. Staying hydrated is important of course. There are some other helpful natural remedies but I will wait to hear back from you.