ctandc,
The simplist way I have found to get my hitters elbow in the slot is to have them throw a bucket of balls sidearm right back to the pitcher. Don't tell them anything else, just let the natural movements of the body take over. Resist the temptation to correct anything until they have throw a whole bucket (40 balls). The natural throwing motion will get the elbow in the "slot." After he has thrown the balls ask him where his elbow ended up before release? He should say the slot. Then when he is hitting, just tell him to throw the bat sidearm at the pitcher. The elbow will naturally slide into the slot.
I have some thoughts on your son's swing, I think it is great that you are working on the correct fundamentals. I can tell you know what you are doing. Please watch the video of Manny posted below.
Watch how when Manny gets to toe touch (slide #56) his bat is angled behind his head, and it is closer to his body. Your son swing is getting "longer" because the arms are getting "disconnected" from the body at the begining of the swing. His backhand should be the the same position at toe touch (hitting) as it is at toe touch throwing. This is also the same position it would be in if he was throwing a punch. Your body knows where it is most powerful. It is our responsibility to let the body get into those positions.