I doubt it is his ankle becoming weaker. More likely his pad is soft from wearing the boot all the time.
If you have ever seen or had a hound right off the track , it is kinda the same thing. Since they generally run and turnout on sand, their pads need to be toughened up (with short walks) to be able to walk comfortably on concrete, asphalt and dirt.
Just my totally useless opinion, but there you have it.