Just noting that a lot of people will read a thread but not have the experience or have read enough to provide a good answer, so do not. That sort of restraint is actually a good thing.
As mentioned, feed and he will grow. He might always seem a bit on the smaller side, but that's not a problem. As long as he's healthy and his bones are good, being small isn't intrinsically bad and, they keep growing. We've had stories here about chameleons that seemed to be almost "miniature" until they hit the supposed age of adulthood and then started a growth spurt.
It's not an exact thing. Biology is not linear, as my boss likes to say.
Finally, life is a learning experience. There isn't a poster here who hasn't learned through making mistakes. You can only go forward with the knowledge you now have.