
You need to make an appointment with your vet to have him examined for hip dysplasia and/or a luxating patella.
People are sometimes under the misapprehension that only large dogs like German Shepherds get hip dysplasia, while in reality it is possible for ANY breed to have it, no matter how big or small.
Small and toy breeds, however, are also prone to something called a luxating patella, the patella being the knee. Its human equivalent would be called a "trick knee".