loss of controll in hind legs-vet says can't find anything...

[Suzie70]

yesterday evening my dog (mix 15 pounds) suddenly started to "wobble" he is allert, has appetit, but it seems that he can't control his both hind legs properly. Some steps he can then suddenly he can't anymore. When he tries tu run, sometimes he just tipps over with his behind side. I touched him and moved his legs, he seems not to have any pain. His stomach feels kind of hard. Would you have any idea what could cause this? He did not have an accident, did not fall, there is no indication where this can come from. Thank you very much.
Certainly, I went to the Emergency Vet yesterday right away, he x-rayed Maxi, but he said he can't see nothing, just that he is not a "puppie" anymore and his bones show signs of Athritis..he thinks it might be "just" age related and gave him some pain medicine...This morning we went to his regular Vet and he checked him, temeprature, reactions etc. everything is fine, he couldn't see any signs of pain or disc problems ( I brought the x-rays). He advised against giving the pain killers, since he seems not to have any pain, he states there is than no reason to mess with his stomach. The little hardend tummy could be from being a little constipated and he gave him a laxative (it worked...lol).
I am just wondering if anybody has a similar experience with their dog, and any tips what else it could be or what else I could do. I mean is walking arround and is interested in everything, even playing, but still sometimes his legs give up on him. I'm just worried, if the Vet's done everything. My animal are my kids, and I am just out of my mind right now. I am even considering to drive him to the LSU animal hospital now. Any thoughts on this issue are so highly appreciated.
Thank you everyone.