i just bought some nyquil the other day. do you know it contains 10% alcohol? that pretty darned much, and of course you could get high off it if you drink enough. mixing that plus something else sounds - sorry to sound like i'm generalizing, or stereotyping - but all too common.
the problem is, worrying about him won't change anything. being fearful won't make him not wanna get off on cold medicine, and it won't make him not do something reckless.
do you think you can work on detaching?