I have bred jacksons before but it has been many years ago (20? time is flying by)- I cannot remember growth rate, but I think I remember they are pretty large by 6 to 8 months. As it happens, now I am raising a small clutch from a wild caught female that are one month old now (wc female gave birth to 4 plus several infertile slugs).
So I may not be much help on growth rate.
I just wanted to say I have always used a heat source with my babies when I bred them in the past, and have been using one this past month since day 1 with my current babies. It is important to make sure they can get away from the basking area to the cool part of the cage, but I think if you set up a gentle thermal gradient you will find that your chameleon basks, and because his metabolism will be up he will probably eat more and grow faster.
My babies have been actively basking and thermoregulating (moving into the warmth and turning dark, moving away from it to cool down with lighter coloration) even during the warm part of the day when ambient temps in my building are peaking around 75-76 degrees.
At 1 month, mine are eating crickets and roaches a little larger than 1/8" long. I expect in another month they will be feeding on 1/4" insects easily- maybe larger than that.
I have been offering a rotation of baby insects including lobster roaches, crickets, mealworms, superworms and last week the baby jacksons were large enough for me to add baby dubia roaches to the menu and they have had 2 feedings of those (1 meal at the end of last week and one this week) and they snapped them up.
Edit- just saw that posted elsewhere his pics and description of his setup. I would *not* add heat to a 10 gallon tank. It is too enclosed so disregard my comments about heat. The babies I have now are being raised in a 2'x2'x3'high screen enclosure so there is plenty of room for a good thermal gradient. Charlie is large enough now for a larger enclosure with more ventilation and some heat. But do not add heat to a glass 10g tank- it could trap and build up in there and cook him- or maybe only very mild heat, like a 25 or 40 watt bulb over one end might be OK, but the cool end should get no warmer than 75 during the day in a tank like that.