Apart from the fact that only one of the two types of stones will dissolve, if he's not going to eat the special diet, surgery is probably your best option to be honest. But only your vet can properly advise you. Don't be too worried if you do have to go for surgery much as obviously it means a g/a and a recovery period. You might try to persist with the special diet however perhaps mixing it with a very little of his normal food. But as this has happened again, you will need to get him onto a preventative diet. Something in his system has caused this to happen again.
Whatever, don't just leave it because if the stone gets big enough, it will block him up (urinary tract) completely, which is obviously life-threatening.