I don't know that we've ever seen Luke and Noah discuss Luke's past with drinking and his kidney problems, have we? As such, Noah may not even know how serious the drinking is. I don't think it's damaging to Noah's character for him to be uninformed and doubt that's what the writers are trying to do.

We have never really seen how much Noah knows about Luke's drinking past. However Luke and Noah lived together and dated for over a year. I remember a scene where Holden has mentioned before(to both) that Luke especially shoudn't be drinking. Plus as Brian said last week, he knows the consequences of Luke drinking. Why wouldn't Noah, who was dating and living with Luke for over a year, know this?

Sometimes people just don't reveal that about themselves to the person they love. I'm not even sure if I will ever reveal my mild addiction to pain killers when I was 12 to the person I later on settle down with but the good news is that I haven't touched that pain killer in 12 years. I think that Holden was just referring to the fact that Luke was underage and also Luke knew what his dad was meaning. If I remember correctly Lucinda told Brian about Luke's past troubles with drinking and we all know how much Lucinda likes to talk about her grandson .

While Noah may not know how dangerous drinking is for Luke, Luke should be on a whole batch of anti-rejection drugs for his kidney transplant. Since they have lived in the same house, Noah should be aware of this and it should raise the question of Luke's health and whether drinking would be a good idea.