"So in your experience could it be that he is having something called cannabinoid hyperemesis syndrome?"

I doubt that.
Cannabis is an excellent anti-emetic.
It is used to treat nausea and vomiting albeit in much smaller doses.

As I understand it, these are symptoms of the loss of hepatic function.
The fact that the symptoms persist after cessation of medication, point that way.

You need to get some water and nourishment in him right away,
Probably by IV drip.

I am not a doctor, so please seek competent medical advice, stat!

Aloha and sympathy

Weezard