I would store it in the fridge, fresh urine is best, but I would keep this sample just in case, keep it refridgerated overnight and in the morning drop it off at the vet. They will test for basics, like glucose, protein, blood, ketones etc.... then if needed spin it down to see if there is bacteria or stones. Those tests can vary, but the first step when they are acting normally is to get the sample in. As long as he is eating/drinking and running around, its all good. Get them run first thing in the morning.

My dog has had cystitis quite a few times, its not life threatening, but its more common in female dogs than male. Its treated with antibiotics and antiinflamatories. HTH!