OK, sounds a bit like you are going to try the "tether" between the boards that I have used for the last 5 years in the "A Taste of Windsurfing" program.
A couple of tips.
First, a a short solid rope will pull the board out from under your son.
Get 2 8-10 foot long pieces of 3/16" line (braided nylon is good) and tie a piece of 3/16" or 1/4" bungee cord in the middle. Buy a cheap 2-3 foot bungee cord and cut the hooks off the ends!
If your Go 170 has the bow eye, or the 2 screw holes under the nose for the tether attachment,
tie the line on at the front of your son's GO 170.
Then, tie the other end to your mast foot (just put a loop around the UJ and tie in a bowline on a bight) .
This way you can encourage your son in a quiet speaking voice (he's < 20 feet away so no shouting or yelling needed).
If you tie it on your footstrap, your son will be doing all the steering and he will be steering you.
Encourage him to try to keep slack in the rope.
1/ Help him to focus on the line, not his hands/feet/ etc.
2/ Keep you within speaking distance.
3/ Allow him to "imitate" what your do with your rig (sheet in/ease sheeting angle/ tip the rig
to steer a little.
He will suprise you!
He'll be sailing up your wake with slack in the rope in a few minutes.
When he is able to stay upwind and set his own direction, untie the tether between the
boards and just sail along behind him.
His progress will AMAZE you!