Well, no doubt you've already seen episode 13.

His next transformation after that is in episode 44 (which is smack in-between two of his youkai transformations, no less). Then there's 57 and 58, the Peach Man two-parter. Finally, there's 66 and 67, another two-parter, this one involving Kouga and Kagura as well.
So far that's all, though I think we see his human form briefly in the second movie as well. Also, without saying too much, there ought to be another transformation happening before too long in the new eps.
If you've got any more questions about what happens in what episodes, Sengoku Jidai has a good episode guide for this sort of thing--that's what I used when I first started downloading IY subs.
http://web.utk.edu/~bborchar/anime_episodes.html