He got it in this book, apparently he can take whatever shape he needs to take now.
In Duel Nature he changes into a Black Bear to better climb on the bridge. I thought it was odd, but he is the great bear spirit and it was just another breed of bear… so eh, okay…
Then in Winterfall, I think, when they were in Rome and then China, Sos adjusted his form to better fight the Vorsook soldiers and the pieces that broke off of them. So it’s been hinted at and shown since early on that Sos’s abilities were changing.