C.P.
The bottom line is that the crate is too big.
I know you feel like you need to buy a crate that he can grow into, but you actually can't. There are crates with adjustable walls in the back so you can make it smaller. There should only be JUST enough room for him to lie down in it with his legs out to the side. Any more and you can expect him to soil it.
Dogs will not soil where they sleep. But they will go in the other corner of their crate if that crate is too big.
If he still goes in a smaller/adjustable crate, then the problem is physiolgical. Talk to the vet.