First of all, you should always take a dog with a sudden and unexplained onset of any problem to the vet.
They will be able to run tests on your pooch and determine what treatments or medicines he may need to help his condition improve.
But you may want to help your dog personally, so you still find yourself wondering, what can you do for a dog who has bad back legs? Other than a vet visit, you can offer a comfortable place for your dog to rest.
If he has trouble standing because of his bad back legs, he will want to lay down most if not all of the time.
You could also try massaging his hind legs gently.
This does not always work, however, as some problems lead to the area to become very tender and hurt when touched.
Other times, the hind quarters of the animal become extremely ticklish, so a massage does more harm than good.
In cases of minor muscle strains, massages can be very beneficial.
This is the first possible solution for what can you do for a dog who has bad back legs.
When it comes time for your dog to move to go to the bathroom or whatever the case may be, you may need to carry him if the situation is severe enough, or at least assist him in getting on his feet.
Watch him closely for signs of severe discomfort and aid him in whatever way you can while being very gentle and careful.
Other than these techniques, there is not much more you can do to answer the question, what can you do for a dog who has bad back legs? If your dog is older, it may be joint issues or arthritis.
There are several forms of canine arthritis, which includes: osteoarthritis which is a degeneration of the cartilage in the joints and is prevalent in dogs; rheumatoid arthritis wherein the immune system attacks the body and usually the joint areas; infective; and idiopathic which means the cause is unknown.
So, you should visit the vet.
Perhaps your dog is overweight, so exercise is in order.
Or, surgical repair of damaged ligaments might be the answer.
Some dogs have even been helped by taking anti-inflammatory medications like buffered aspirin.
Some dog parents have tried and believe that alternative or holistic remedies like acupuncture and herbal medicines are the answer for a dog that has bad back legs.
On the other hand, perhaps your pooch was injured and has a strained muscle or it could even be a thyroid issue.
What you want to do right now is stop speculating.
Since you are not a veterinarian, you cannot diagnose your dog.
Take him to the vet to identify the real issue and how to treat it.