I agree, a HUGE improvement that you should be VERY proud of.
However, 9 year old horses don't grow, as Kayty said. Even malnourished mature horses don't gain height when put on a proper diet. If you were just told his age, it's very possible that he is younger than you think. Do you know how to check his teeth for a guess on his age? Or you can ask your vet/equine dentist at his next float. I'd look it up, I hear it's pretty accurate. =]