My dog has a similar problem. It has escalated to a debilitating condition. The best advice I can give you is to take him for a complete examination including x-rays. It's an expensive process but worth it if a condition can be identified while there is still a possibility of treating it. Part of my dog's problem is that she is very overweight. If your guy is overweight, the first thing to do is try to get the weight off. If the problem is just muscular weakness, exercise may be the best answer. The best first step is the vet.