Back legs

My dog can't get up the stairs anymore or jump onto the settee, he loses his balance every so often and when I go to pick him up he drops to the floor and his back legs go, do you no what's wrong with him?

[Aleda M Cheng, D.V.M., C.V.A]

Since your dog is young the problem may be due to a congenital, developmental or traumatic injury.  But there could by other possibilities also. You did not give much history so I will just list some general possibilities.

The problem could be due to:
a bony disease of the hips or knees (such as Legg-Calve-Perthes disease, hip dysplasia, or medial luxating patella's).  Or, other possibilities include: disk disease of the spine, degenerative disease of the nervous system (such as a degenerative myelopathy), or a seizure disorder,  cerebellar hypoplasia,  or other abnormality of the brain, or Lyme disease and many others.

This problem truly requires a veterinary exam and probable X-Rays for diagnosis and possible treatment.   Please bring your pet to the vet ASAP.