I think what is happening is pure fear mixed with placebo.

When I first started (or at least the first 5+ times I smoked) I was always worried about knowing when to stop, or being to high.

What I would do with him is pack a bowl of mids, and have him take a hit evry 5-10 minutes until he is at a comfortable spot, high-wise.

Remember how many hits he took, and have him take that many next time.

Theoreticly, this will determine his good-high spot.

Also, don't be afraid of acting liek a dumbass so he laughs.

If he can focus on you being funny, other then being high as hell, he will start to understand the whole point of being high.