[msdany]

My 1 yr chihuahua is complaining of pain after falling off couch. I know its not broken he letsme move it but he won't walk on it, what can I do to help him?

[Alty]

Take him to the vet and find out what's going on.

[Aurora_Bell]

Agree with Alty here, only a vet can tell you what's going on. Could be muscle damage or torn ligaments.

[Sariss]

To the vets in the morning. Chihuahuas are prone to luxating patellas (dislocating knees) so it may have popped out and is causing your dog distress.

[shazamataz]

Patellas are a possibility with any small dog, but if OP has been moving the dogs leg, it would have popped the patella back into place.
When the patella is out the dog physically cannot put its leg back down, it doesn't choose to hold it up. You can manually click the joint back into place and the dog can walk again.

My mothers grooms a poodle with really bad patellas, all the dog has to do is twist wrong and the knees pop out, she just lifts the dogs front legs off the ground making the back legs extend and it pops right back in.

I would keep your dog confined either by crate if you have one, or just by keeping an eye on him and making sure he does not try to run around. Give it until tomorrow and if there is no improvemtn make an appointment with your vet.