Make sure he does drink alcohol along with Nyquil. I would not mix alcohol with Nyquil. That can be dangerous. Can damage liver and I heard that when you "combine alcohol and ANY downer (Nyquil, in this case), you run the risk of seizures, coma, and death."
Also, you shouldn't mix NyQuil with Tylenol. Tylenol is a brand name for acetaminophen. You should look into information about a drug before taking it, prescription or not.