I wouldn't recommend getting him high or even simply giving him smoke, sick or not. It might just make his congestion worse, and while weed smoke helps some people (not all, I was one of the ones it made asthma worse in) with asthma and congestion, pets are uniquely different. They also sometimes react badly to smoke of any kind. You wouldn't want to risk making him sicker. And a pet with wheezing or asthma trouble is a pitiful sight. It'd be even more pitiful if you knew you'd caused him to be that way, right?

Sounds like he's getting better with just the treatment you're giving him, so just keep up the good work. Excellent job!