Shih Tzu also commonly get the same disease as Dachshund, IVDD. Basically their stature puts too much pressure on the spinal discs and things like jumping and stairs really don't help because of increased impact.
I do not know if they are prone to disc drying/ calcification like Dachshund.
If this IVDD then there is surgery. It is not cheap but it will help and will probably get him mobile again. If he is still using the back end some he probably didn't rupture a disc, but maybe herniated it. This may be difficult to see on X-ray. If you have a local veterinary school this might be the best place to go, they will have all the technology and knowledge to make a diagnosis and will be able to do surgery at the most affordable price.
ALso make sure he is a proper weight, being overweight increases many joint issues. Also no more jumping up and down, you might want to put in a ramp to the couch.