You should take him to a neurologist and even then you'd be surprise at how many are not familiar with SM, or have only head of it. Cavalier.org has a list of neurologists with ones in red font that are familiar with SM. My puppy started showing signs at 5 months old as well. My vet misdiagnosed him as have allergies and then a sprained back. It was not until I was dissatisfied with the vet that I started researching on my own and came across SM. He had an MRI a week later which confirmed it.
That said, DO NOT assume that he has it unless he is bunny hopping when you walk him and screaming for no reason. And if he does have it, we have had a lot of luck managing Jack's pain with diet and acupuncture. And he is going to be a year this Sat., and honestly he is almost a normal dog (albeit expensive with the treatments and compounded medicine with side effects that sometimes cause there own set of problems). Also, I would get him a step in harness immediately as that will relieve pressure from his neck (most of the dogs have skull malformation, so it is a good idea anyway).
Mary Ann (Jack Sprat's mom)