The alcohol would be the most likely culprit in the brain damage department. It's really one of the most neurotoxic of all "substances", though chronic benzo abuse can cause cognitive decline.
If he was using xanax and alcohol, that's the most likely cause of the blackout right there, though xanax is notorious for causing blackouts on its own in high doses.
Can't speak for the cough/cold medications, which are taken for DXM,(dextromethorphan) which can cause a very heavy psychedelic trip if taken in high doses. DXM has its own "culture". I can't speak for the DXM experience, though have read up on it.
I've only taken medications containing DXM for the purpose it is meant to be used for: cough suppression, for which it does an okay job in normal doses.
It's the mixing benzos and alcohol that scares me the most: not just because of the blackouts, though people do really scary things during blackouts, and both xanax and alcohol can cause aggression in some people, but because the two drugs potentiate each other, leading to a very high risk of overdose.
"Educated" benzo abusers warn against combining benzos with alcohol for this reason.
Note: I am not talking about taking .25mg of xanax and having a glass of wine, here. That'll just put an inexperienced person to sleep.
However, if you are talking about the usual "box wine" boxes, those are 4ltrs, which is a huge amount of wine, let alone 2 of them, and box wines are usually high in alcohol. He then took a substantial amt of xanax, a substantial amt of DXM, and the Red Bull, probably because he was getting sleepy from the alcohol and xanax and wanted to stay awake to enjoy the "trip". With the amount of downers he had in the system, the red bull wouldn't make a difference. In the future, if he comes home in that kind of state, make sure he goes to sleep on his side, with his head turned to the side, and check on him to make sure he stays in that position. That will prevent him for choking on his own vomit if he pukes in his sleep.
Gak. The kid needs help. I assume he lives with you? Does he own his car? Whose insurance is he on? If yours, get him off of it. If the car is yours, stop letting him drive it.
I'm sorry you are dealing with this. This is sort of a nightmare situation and very scary. Not just because of the risks of xanax and alcohol abuse, but because if he is abusing DXM frequently, there is a risk of him becoming involved in that culture, which offers very little in the way of good things.
LSD, Peyote, Mescaline, Mushrooms, Ayahuasca, etc., used under supervision, do offer the possibility of self exploration. recent research is actually starting to show clinical signs of this, but DXM is bad news.