Where I can't really help him is how should he decide to play the straight drive, or a forward defensive shot? I don't really understand what the decision should be based on, since to my mind they could both be played to a similar ball.
I agree with you, the straight drive and forward defence are essentially the same shot, just with different power and follow through.
Without seeing your son, the usual problem with the kids I coach (and many cricketers) is they see something pitched up and their eyes light up. The head lifts, the foot doesn't get near the line of the ball, and the bat becomes angled instead of vertical, and they try to hit the cover off the ball. Getting bowled is very likely.
If you can keep those 4 technical points where they should be, and time the ball correctly with some power, then you have a lovely straight drive.
If you have access to a bowling machine, get him to practice a correct forward defence for ten balls, then drive the next ten with the same technique, but more intent. Repeat.
I personally find hitting straight drives off the stumps the easiest shot to play, as it requires very little head or foot movement to get your eyes over the ball. I'm rarely bowled, but usually caught with poor timing.