I recently bought some little filters called Tar Stoppers from the cigarette store (about $2.99 for a pack of 30 - and they work on 4 to 5 cigarettes, plus I read you can clean them out.)
The filter out a lot of the tar & some of the nicotine. My son will use them - sometimes.
But, you can actually see the tar & gunk in them after just one cigarette - it's pretty amazing. It's thick, resiny stuff.
If he will use them, it would probably help the coughing.
If you can't get him to stop the pot, maybe get him something safer to smoke with like a vaporizer so the smoke isn't going in his lungs.
Same with the cigarettes. I don't think e-cigs are safe, but at least you're not getting the tar in your lungs.
I know the best thing is for him to stop, but that's not very realistic either. We just have to do what we can to mitigate the damages.
Also - I read that the tar is what makes them metabolize the Clozaril faster - not the nicotine. And even in these early stages, I'm seeing the drug be more effective on my son if I can get him to use the e-cig & the filters - even if he has a few without the filters.
They even have a story online about someone on levels like your son who went into the hospital. They gave him a nicotine patch, but the Clozaril still went to toxic levels because he wasn't breathing in the tar.
If you get him to drop any of the smoking - or you get him to smoke in a cleaner method - keep an eye on the Clozaril too.