Hi Lupi. I have treated spinal injuries in dogs for 11 years. I even have a Dogue that i rescued from being PTS when he was 7 months because of a server spinal injury.
I am first and foremost a canine hydrotherapist and secondly i invented a harness for dog mobility issues. Forgetting about my dog mobility harness for a moment - i would suggest swimming as a fantastic way of building up the muscles in the dogs back. I actually have a video i took of a dog that suffered a spinal injury - she was operated on and swan for some 8 months twice a week. I'll put the link on for you below, but before you start thinking you cant afford twice a week for 8 months - i assume you have a dachshund from your avatar picture.
You can buy a life jacket from ebay and swim your dog in the bath. Get some advice from the vet first please to see if this is the right course of action for your little baby. Heres the video: http://www.youtube.com/watch?v=FQKzo9ETM2c