What can you do if your dog can't get up steps due to leg pain?

First I would strongly recommend making an appointment with your veterinarian for your dog to have the source of the leg pain identified. In some cases, the pain can be completely cured through surgery to correct the underlying source of the pain; in other cases, such as chronic Arthritis, your veterinarian can prescribe pills to alleviate the pain that are safe for your dog.

Second, if your dog is small enough, you can always choose to simply carry him up and down stairs, or to lift him up to places he wants to go but can't jump to any more.

Third, you can install an incline over part of the stairs, with traction on the incline, so your dog is more walking up a hill rather than trying to go up steps.

Fourth, there are some instances where you can put a special ramp in for your dog, such as by stairs on the porch; if you make it wide enough, it will also be handicap accessible for humans, which is becoming more useful and desirable.