First, welcome to PT!

I'm glad you've already been to a vet! It doesn't sound as if it has become really bad yet, but it can, and in that case you have to act quickly, because if he's blocked it can become fatal within one or two days. As I said, Fister has had the same problem, but he's back to normal now!

The first time, Fister had a cahteter put up and was flushed through, he then had some antibiotica and was only allowed to have Hills Prescription diet S/D food, luckily he liked the dry version but would not touch the canned food. We can only buy this at the vet. There's also one called Specific FSD which works the same way. It is supposed to dissolve the stones, but he didn't care much for that. Recently, out vet got a new one from Royal Canin called Urinary programme LP30. They all have a low quantity of Ash, Calcium and Magnesium, which are VERY important! Fister definately prefers Hills (dry food) to the others.
Don't give him ANY other food, (I know it's hard) but take him to the vet again in a few days if he still has problems!

It's friday, so you should find out whether you can get any of these (we can not get them in ordinary pet shops in Denmark) before your vet close down for the weekend! It's so important!!

Also, see if you can make him drink as much as possible (WATER), this should also help a lot.

You're welcome to contact me by PM also, if there's anything you want to ask!

Good luck!! Fingers and paws crossed here!