Nyquil, most mouthwash, vanilla extract, and many other over the counter non traditionally products not thought of when thinking of alcohol do contain a significant amount of alcohol.
I know of a woman in my home group that relapsed with vanilla extract. She started drinking that because she "liked it" but then a bottle turned into her buying it by the case then back out drinking regular alcohol.
An alcoholic will drink whatever will get them drunk. Things that would sound completely disgusting to a normal drinker do not even phase an alcoholic wanting a drink. So it would not surprise me if he is seeking the alcohol rather than the relief for cold symptoms.
One way to find out is to offer him some cold tablets such as tylenol cold and flu in replacement for the nyquil. If he refuses then he probably is drinking it for effect rather than health.
A concern I do have is if he is drinking nyquil he may be to a point in his disease that he could suffer withdrawls if he does not have alcohol in his system. It would probably be best to be aware that he may need to be detoxed by a doctor.