It's not because of any big reason, actually. MC isn't changing back yet because
1. He's still injured and changing back into his human form hurts too much (and would rip the wounds open)
2. He's guilty of punching the ML so until he can escape, he's pretending to be a normal puppy (though he's not doing a very good job and ML already knows )
I'm not sure about what the other comment is about tho, I'm still only halfway in the novel

Dude, just transform. Do you have a bald spot on your ass on your wolf form or something???