I'm pretty sure it was only ever the outside covers of the books that ever referred to "the Animorphs and Ax." Within the books themselves, as far as I know, he was always treated as an Animorph except for the times that he would alienate himself.
A good example is the fact that, on the outside cover of #48, David is referred to as "the sixth Animorph" whereas whenever he has been mentioned within the actual books he was called "the seventh Animorph." Obviously KA was counting Ax and Scholastic (who writes what appears on the book covers) wasn't.
And if you want my own personal opinion on the matter, Ax is an Animorph. He's a part of the group, I don't care if he isn't human.
