Could be the seasonal changes are having something to do with it. My chameleons all seem to decrease the amount that they are eating in the cooler seasons. They are kept indoors most of the year but the house is not air-conditioned so there are fluctuations in their temperatures.
What are the temperatures in his basking area and the other areas of his cage?
He could be slowing down as he approaches adulthood too.
However you are saying that he is on a hunger strike....do you mean that he isn't eating at all?
For years I have fed my babies moderately...well fed, but not overfed...and they grow much slower than most people have reported that theirs grow. They still reach a full adult size though....it just takes longer. However, I've never had one quit eating completely....but I'm betting that as long as it wasn't for a health reason, he will still reach adult size.