First, before you even begin to train your Dane, you must get his hips and elbows cleared. You may need to wait until his growth plates are fully formed, so absolutely no mobility training until then, only basic obedience.
Considering he is a Great Dane and is going to be used for mobility purposes, he is going to need to be completely sound before even thinking about mobility work, especially weight bearing. At such a young age though, it will be hard to tell how his joints will form, but he should be x-rayed as a pup and as an adult. It is also recommended to not neuter him until at least 14 months, to make sure growth plates are fully formed, to avoid injury from service work. I did not heed this at the time of my dog's neuter, and am kicking myself, because my dog technically has more of a chance of injuring his cruciate ligament, which could possibly retire him from service work.
I would also highly suggest finding and hiring a positive reinforcement service dog trainer, one who is well-versed in actual task training and mobility work. Without one, with your disability and inability to walk well or leash train a young, VERY large dog, it will be near impossible to properly train him.
Start searching for a trainer now, and go with the one that is the most experienced in training for service work. Feel free to PM me about any other questions, I'd be happy to help.