If his heels have grown forward(underslung) then his toe has grown forward too. Thus his angle is not the same as it was when shod. Pulling(rasping) the heels back so they are where they belong will lower them. Rasping the toe from the top/front will help encourage the heel to stay under the back of the hoof.
I strongly suggest you look over a few barefoot sites and familiarize yourself with the hoof. This one has good pics and info and links to other respected sites Amber Mohr's Barefoot Hoof Care