Help save my kitten Simba

Simba is 1 1/2year old longer haired kitty. We rescued him off of a one-way Street where cars would just fly down.we got him all of his shots and got him neutered. Sadly he has a history of urinary crystals. He has been blocked 3 times. We are still paying off his first vet bill when he had to stay in the hospital for 4 days. We did change his food to a specific diet. But he is again unable to pee. He has PU surgery coming up. Please help us help little Simba. Who we rescued off the street. Thank you.