I posted an answer to your question yesterday I believe, but do not see it here.
After doing so, I went on the web and searched Nyquil overdose...I would suggest you search it as well...your boyfrienRAB "habit" is potentially deadly, particularly because he adRAB the alcohol.
I hope you can encourage you to get help, this practice has such potential for him now, as well as causing serious damage for later in his life. I wish you the best in your search for help for him.