First, let me say I am no hitting expert. All I have is experience with one son. Also, you didn't mention the age of your son, so what I have to contribute may not be relevant.
My son is 15 and we are told he has good fundamental mechanics, which I attribute to professional batting instruction he had when we was 11-12 years old. Last spring he developed what I would describe as a loopy swing - a much longer swing. I used the video camera to show him what it looked like - he could see that he was off, but he didn't really know what he was doing. After watching himself bat on video, he pulled out the T and worked in the garage awhile, then decided that his problem was with his weight distribution. Since then, no loopy swing and his batting average has increased dramatically.
I'm now a fan of: (1) good batting instruction, if that is available/affordable for you and (2) owning your own T so the player can work it out when they get older. Essentially my son was able to self-diagnose by having both.
As for drills, I will leave that to the folks who understand hitting mechanics much better than I do. Good luck and enjoy!