In a 9/10 year old dog with those vague symptoms, I'd be more likely to suspect disc disease or other progressive degenerative joint disease. Stiff joints, weak limbs. Not uncommon in older dogs. Even if it is SM, I wouldn't do an MRI because the main reason too d that would be to absolutely confirm SM for surgery or breeding purposes. In an older dog with not very severe signs, I'd lean towards just managing the pain and discomfort and not stress too much about definitively diagnosing. REally, you just need to speak with your vet and see what they think. They may be able to easily diagnose a managable problem with just palpation or x-rays.
Treatmenet for SM is not easy, not definitive and verry experimental at this point. Surgery is not a cure and is often unrewarding and unsuccessful, with relapses always possible even if they do improve initially. medical management is aimed at decreasing symptoms and discomfort with pain meds, and/or diuretics to decrease cerebral spinal fluid which is thought to be part of the cause of the disease syndrome.
It sounds to me more like your dog is just getting older, having a harder time getting up and moving around. All consistent with age related problems that can usually be well managed with Non-steroidal antinflammatories like Rimadyl. Sounds like he's responding well to it?
Dogs are not our whole life, but they make our lives whole.