His childhood experience conditioned him to have a panic attack when exposed to a negative stimulus. The stimulus is, of course, the smell of cannabis. The smell was once a neutral stimulus without a response, but after the incident, his brain paired this harmless stimulus with that of a psychological and physiological response: his panic attacks.

This is different from a "contact high" in which enough THC reaches the brain to give you a high.

There's a behavioral-approach therapy known as systematic desensitization. Not sure if it could work in this case - try it out!

I assume his traumatic experience was getting really high and having a severe panic attack?
:stoned:

*edit* typo