I'd probably start out with the micro adrenaline rather than the micro midas. They both go from 18 to 28 inches but the adrenaline can be had in higher draw weights. Another pluss with the adrenaline is that it has about the highest available speed for a youth bow and with low poundage bows you need all the umph available.

I am waiting on one for my son and he is 11 years old but big for his age like your son. He decided on this one after trying out the Alpine micro & Parker challenger. Dollar for dollar it's about as good a vlaue as there is in youth bows.