Heck, his life may have ended via suicide. Really, becomeing an Animorph was the best thing to ever happen to him. It gave him a purpose in life, and close friends. Though in the end Rachel dies, and he separates himself from everything else good in his life. His friendship with Jake is shattered, and he apparently doesn't feel like going with Ax or moving in with his mother.
If Tobias actually got his Human body back after book 13, instead of being a morph capable hawk, then I could definitely see him going to drugs. But the hawk thing gets in the way of that.