When you get a chance, give your vet a call. There are diets available that can help bolster feline urinary health.

There IS a more permanent solution if it does recur. It's a surgery called perineal urethrostomy, and what is essentally does is make the urethra wider so things that would otherwise obstruct it can pass. I have no idea how much that costs though.

What grade was his heart murmur?

I'm really glad he's okay. He's probably feeling about a billion times better right now. How scary that must have been for both of you!