Will your dog jump on and off the couch if the steps are not there? If he won't I would consider removing them for a while. I have a dachshund that has had sugery and physical therapy but the surgery was too late to help (I came home to him paralyzed on the floor). If your dog is not in severe pain or paralyzed, I would just watch him carefully and take him to the vet ot be checked tomorrow.
One thing that I would change is to work on discouraging him from climbing up or down stairs or any jumping(going down is worse because it compresses the spine). Train him to wait for a person to pick him up. Also always support his back end when you pick him up. Another factor since your dog is rather young, is he overweight? If so, losing weight will definitely help his back.
I have known of MSM/glucosamine supplements to be very helpful for older dogs with arthritus symptoms and I doubt that it would cause any problems and might help.
Edited to add - if a dog's back goes out to the point of paralysis, the sooner surgery is performed the better the prognosis. In the case of our dog he was probably injured for too long before he recieved surgery. We don't know how long it was because I came home and foun him paralysed.