haha why did you buy him a juno 106? How much did you pay for it?
Disclaimer: I'm no tech
does the problem effect keys completely randomly or do selected keys not work? If the problem is completely random then it's probably the Chip problem. If the problem only effects a key or certain keys (for example the middle C key and no other keys), then the problem is the keys connection.
I've had problems with 106 VCF/VCA chips and in my case the effected voice would trigger sometimes cleanly, sometimes come out distorted and sometimes wouldn't play at all. (perhaps this could explain why the test worked?)
The course of action you should take depends on
How much you paid for the synth.
Whether you want to keep the synth if it involves an expensive repair. You need to find a tech/vertify the problem, then if it is the VCF/VCA buy six of these http://www.analoguerenaissance.com/D80017/
. (my juno currently needs repair because I only replaced 3 of the buggers (pardon my obscene Australia-english) and another has gone.
Then you must decide whether to keep/repair the synth or just sell the synth for repair and perhaps not lose money or not lose to much(thats why I want to know how much you paid?). Then go and buy your son a roland gaia
. A good learning synth that's also cheap and reliable. The 106 was my first synth I bought when I was a poor adolescent student and it blew up, I didn't have money to get it repaired or buy a new synth and went synthless till I left school !).