I just found and read about his accident and am so sorry
but what a lucky guy he is to have you.
All the best and here's a few sites for you to look at - even if only for the pictures so you can make your own wheelchair - and have you tried little diapers on him ? I read in your post he was getting a rash - is that better ?
That's a shame those carts sell for so much - so much for caring about the DOGS that need them when they're priced out of this world ....I swear if I were an inventer or mechanically inclined and could make them myself - I'd give them away practically free because any dog that needs them has already wracked up a ton in medical bills -
http://www.dogmobile-online.com/ http://www.handicappedpets.com/