Hi. I'm Dr. Lee and will do my best to help.
What breed of dog is he?
How long has this been going on?
Thanks for the update.
What you are seeing is likely disc disease (intervertebral disc disease). He needs to be seen by a veterinarian right away, and get x-rays, and most likely will need MRI and surgery. If this is not done right away, he might never regain use of the back legs. Please do not delay this.
Please let me know if you have any questions or concerns. Thanks.