Its a device that Ken Griffey used to market I think. It is a "T" that has a tube above and below the "T". The idea is that the bat needs to travel between the two tubes and hit the ball sitting on the tee. I think it is designed to encourage a slightly downward swing plane. We have on laying around somewhere in the backyard. It has not seen action since little league days.
After re reading your initial post the instructo swing may be helpful in developing muscle memory that could help your son. There are other ways to go about it for sure, but I do not believe the device would hurt your young player. When mine were 9 they did use it for a while. After they developed the muscle memory it has started to turn to rust, but it was a decent tool and not very expensive.
If you pay the shipping I will send you ours. It just needs a replacement "T" I believe.