tab, I am so sorry you are going through this. I dont know if triple C's is physically addictive like alcohol, or the opiates. I am guessing your son is similar to mine in that he will use whatever is at hand to get high. Since the cough medicines are OTC they are easy to get. My son has told himself in the past he is not an addict because he is not hooked on heroin or something like that.... but he will use anything including the cough syrups, and he has also huffed the small whip cream cans. To be that is an addict. I dont know if anyone can tell you what to do.... I think part of the process is to get clarity with yourself about what the right thing to do. That can be difficult. I do recommend that both you and your partner go to alanon.... that has been a huge huge help to me.
I know kicking them out is really hard and it is very scary to have them homeless. I have both kicked my son out (at which point he went to friends) and have let him be homeless. Him being homeless was very hard on me and I admit I am doing what I can to avoid that again..... but sometimes it is the only thing you can do. It certainly is a wake up call for them. When my son was homeless we always told him we would help him get treatment and he did that several times. So once they realize they really are on the street, and they live on the street, that wills sometimes propel them to get help.
But of course there is no guarantee and it is scary to have them on the streets. I will say that at least with my son he showed us he is a survivor and he found ways to survive... and in many places there are lots of services for the homeless.
It sounds like you are your partner are not on the same page... and it is important you talk to each other and find ways to deal with this together. I am guessing from your post that he is your son and not her son? That makes it harder. But you need her support as you go through this.... and it is definitely not worth letting your son hurt your relationship.
Unfortunately there is no one right answer..... I think ultimately you have to find the best path for you. I will say your partner being harsh on him is probably not the reason he is using. He is 24, an adult and he is making his own path at this point. I dont think there is any rhyme or reason why some kids become addicts and some dont. As they say in alanon, you didnt cause it, you cant cure it and you cant control it.
TL