It might be Medial Luxating Patella
In which the kneecap, or patella, dislocates outside of its normal trochlear groove. Puppies to young adult dogs to older seniors with MPL are seen with an intermittent "skipping" or "limping" gait. The owners describe the dog as pulling up the leg for several steps before returning it to the ground and then resuming normal ambulation with no sign of lameness.
Especially common in many toy breeds, but an increasing number of larger dogs are becoming affected.
Dislocation, clinically referred to as luxation, can occur on either the medial, or inside surface, or the lateral, or outside surface, of the knee. This abnormal displacement of the kneecap results in pain, cartilage damage, and arthritis. There are varying degrees of severity of this disease.
Surgery may be needed, but not necessary. To learn more: http://www.veterinarypartner.com/Content.plx?P=A&A=2448
it might not be this cause you haven't said anything about going back to normal. hope this helps :)
Your dog needs to be examined by a Vet and get an X-Ray. You should not apply pressure in that manner in the future as it could make the injury worse. The Vet knows exactly how to apply any pressure during an exam to avoid further injury.
If the shoulder is displaced he would still be able to apply weight to it.
Please take him to the Vet for an X-Ray and let us know how he progresses.