The ataxia that you mentioned - stepping over in the rear is a neuro symptom (as mentioned above). It is not, necessarily, degenerative myelopathy. You should have him examined by a neuro vet (ask for a referral) and at a minimum get an x-ray which can look for degenerated disks.
Most vets will see a GSD with rear end neuro issues they'll diagnose "DM". This is why you need to see a neuro vet.
For DM, it's a "rule-out" disease - meaning they rule out all other causes. The rule out diagnostic, which is an MRI, is not inexpensive. The genetic tests for DM (there are 2) are not statistically proven for German Shepherd Dogs, so to me they aren't worth a dime. If someone tries to get you to test, ask to see the data for our breed - there is very little.
Don't buy a wheel chair for your dog just yet... Assuming it's not a disk issue, you need to keep the rear end exercised and active. Walking on uneven ground, exercises where the dog must step up in the rear (stairs, step over exercises), and the best one - swimming - can all keep your dog going.
Last, from the point that we noticed those issues in our DM dog to euthanasia, we got 3 years. The vet "specialist" gave us 90 days on the outside.. So don't give up.