An arched back is a symptom of intervertebral disc disease. Spinal issues like this where a disc is actually bulging resulting in inflammation around the spinal cord are very painful making it difficult for the dog to walk. Because it is the back and the spinal cord is a very delicate organ I recommend for tonight to keep him in a crate or small area. Prevent running and jumping and force him to rest. Tomorrow he will need to see a vet. The vet can check his back for tenderness, take an xray, and get him on prednisone or other antiinflammatory as well as pain support. Rest will still be needed for up to 4 weeks. If at any time you notice weakness in the legs, inability to urinate loss of bladder control or paralysis your dog will need to see a vet immediately.
Dogs arch their backs for more than one reason. Kidney or bladder pain, abdominal pain as a result of an obstruction or abdominal masses can cause him to arch his back. We had a dog who presented with what looked like a back issue that ended up having a blood urethra and was unable to empty his bladder. So if you notice any bladder issues at all take him in tonight.
If he developed bladder stones then he develops crystals too. This is more of a reason to have him checked especially if you are not seeing him urinate a good stream or having seen him do so in the last few hours.