Even if you get a second hand Ace250, you could say it looks like it's the one Sandta used to use himself lol. You could tag it as for your son
a n d you as a present, so that way you both get to share it ,... and you get your first detector that way too !
Now as for the young fella, well you could hip mount the control box for him and reduce the length of the telescoping shaft ,... this will do two things for him, reduce the weight of the unit, and make it a useable length for him. It's a bells and whistles unit anyway, so he can operate it without looking at the screen too much, and you could belt mount it in a way that enables him to look down and see the screen if necessary, and also so he can use his non dominant hand to press the pinpoint button when in use in an easy way to see and do.
Eventually your son will be big enough to use the unit in the fully assembled fasion as per normal.
There are a few of us on here that have posted good information on how to use the Ace250 in a very effective manner and you can find that information on this site with a simple search.
Hope this infirmation helps.
And maybe some on here could even have a detector for sale.