Nah, if you really wanted to improve him you'd have promoted him to the majors and have him start until he pitched a good game and then demote him back to rookie league.
I cant get pitchers to develop in OOTP and I try to do the stable one level at a time. WHile I've seen AI pitchers pitch 3 games at AAA, then 4 in the majors, then 2 at AA, then 3 in the majors, then 2 in AAA, then 1 in the majors then back to AA for the rest of the season. ANd they develop? Yep.