My son tells me straight to my face there is nothing wrong with his use of Marijuana, despite the fact that his father (my ex husband), has been in rehab 5 times that I know of for drug and alcohol abuse.
It's rife in my family.
If I were you, I'd just let the argument go. Deep down he knows he has a problem, I have no doubt. The addiction needs to defend itself. It's hard, but it's not up to you to tell him 'how it is'.
'how it is', is obviously different for him.
I'm currently re-learning knowing when to 'let things go'. Doesn't mean the other person has 'won', it means you accept they have an opinion/perception, and it's OK if it's different from yours.
The fact is anyway, whatever reason he used to justify the relative drinking is a moot point. The drinking starts for one reason, and changes over time as the addiction takes hold and compulsion takes a grip.
The most reassuring thing is, you always have support here.