If he is straining to urinate I would be worried about stones blocking the passageway. Talk to your vet about immediately putting him on Cranberry Extract. Here's a link, but you can get it at pet stores and even the human version at health stores - take the human dosage divide it by 125 and then multiply by your dog's weight. Most of the cranberry juices you see have sugar (not good), but you can buy 100% cranberry juice without sugar, but it doesn't taste very good so I would just go with the cranberry extract powder.
Also Vitamin C has been known to dissolve stones in 24 hours.
http://www.jeanesholistics.com/Cranberryextract.html