This thread is bringing back many memories of my struggle with Marty. If he still walks outside any you need to watch and prevent him from dragging the top of his toe nails away. Once they are ground into the quick they won't heal as quickly as he grinds them away.
I glued Soft Claws Nail Caps over his existing nails and replaced them when they wore down or came off. This may not be an issue at all I just wanted to alert you to watch.
Soft Claws Nail Caps for Dogs
The Dorsi Flex like all these contraptions is both good and bad. The piece that goes on his upper leg will rub and cause sores. The boot isn't really the same shape as a dog foot and will also cause sores. If you do get one I would suggest using Vet Wrap under both pieces to help prevent the sores from starting. Also get a bread tie and put it about the middle of the straps to keep him from getting his leg trapped between them.
The whole thing adds weight to his leg and he will drag his foot more but at least he has protection. It probably gave Marty 2-3 months more of mobility that he wouldn't have had without it.
It is a tough road and I wish you well.