I've had a similar issue with cats, and twice with dogs.
Depending on the type of crystals, the dog, and other circumstances, sometimes they do not result in stones.
The components for some crystals are naturally in the urine, and in some cases they come out of solution.
Yes, I've heard that about the special Pro Plan. But, I've never used any of them. Well, I did use Hills once, for a short time, when Jet was sick, but it made him sick so...
In our case, we were told to give the dog a drink which balanced their ph, and something else in the diet, but it's been too long to remember what it was now. Also to give him plenty of water. It worked. Even after he was placed, the new dog dad kept in touch with us, and Tig never had any problem. That said, he was on the balancing liquid thereafter, but ate a normal diet along with it. Mind you, he was tested for bacteria/infections 3x's a year.
I know that you are not sure if it will lead to stones, etc. but it's also pretty radical to put him on one of the special foods forever, at his age...
If I had to go that route, I would probably opt for the Pro Plan.
Please let us know what you choose to do, and how he's doing.
Love All Life, Thank You For Posting! :)