I have had a couple of males block up many years ago, but recovered with vet's help with no further problems, and I also lost a couple over the many years from this as well. I too was told it was related to diet and that cat foods with high ash content contribute to this. (keep in mind this was probably 30 years ago) I was told too that if they are not drinking enough water as well, this may contribute too - and having your kitty on IV's for fluids is keeping him hydrated and hopefully the excess water will help him flush his bladder of the crystals. It will also keep his kidneys and everything else functioning.
Poor guy, I just HATE this problem. I changed cat foods years ago and no longer feed Purina or some of the 'store brands' and have never had another problem.
Wishing the very best recovery for your kitty, and if you do surgery, hopefully that will resolve the problem! Sounds like a reasonably good success rate.