It looks like he just too tall to be able to stand up straight in that box to me.

one of my friends has a cur dog he uses to catch cattle with and he's been like that scence he was a pup and he's still goin strong at 8 yrs old. He keeps him on joint pills idk how much help that does but it mite make the whole difference and he's plum crippled lol but it doesn't seem to effect him