Gosh, op he's lucky to have you in his corner, but as others have said difficult to do anything if he doesn't want to change and telling him he might become addicted too often is likely to see him shut down. After all he's taking it, he knows what it's like and addicts lie, especially to themselves.
If it helps my advice would be to try and talk to him, but also listen to him an pick your battles.
He might not believe that he could ever become addicted, but maybe he would acknowledge that its possible for a recreational user to become addicted without realising until it's happened and that could plant a seed of doubt.
Maybe ask him if he's started having the most vivid awful nightmares and if he realises that's due to the Heroin.
Or if he's noticed a problem with his sex drive and whether he could maintain a relationship on it.
What if he got a different batch that was stronger (might not even notice depending on what it was cut with) or cut differently, what if he then became addicted or OD'd? Or if his tolerance increased over time? If he acknowledges this could be possible maybe exploring just how little help is available could give him a reality check. Would he want to addict himself to methadone - even harder to get off. How would he explain that at work? It would be noticeable. What about whilst he was waiting for treatment. Does he realise how much his life could spiral in those 6-8 weeks?
What about random drugs tests if driving?
The effect of a criminal record if his dealer is under police survelience?
Maybe pass on your take on high him, ok in a brother, but not in a relationship / an employee, etc. You said it was noticeable, this may surprise him.
Feel free to call BS on any self enabling talk. You don't have to enable or ignore you can call him out on it without even causing an argument and plant doubt just by stating the obvious or replying honestly to conversations he starts.