My fiancee and babys dad to our two month old son is addicted to shooting up roxies .... I need help!!! ?

I just need advice... My boyfriend shoots up roxies .... Its like a broken record, him saying he cant keep spending all our money on this, he hates it, its not him ... I know he wants to stop this and I want to help him. Our son needs his dad. But its not fair to our son or me for him to keep putting us second as well as our son not needing to be in a sitiation like this. ( he does not do it at our home) I am ready to give up I feel like ive done all I can to help him and know he needs to help himself.... But he is insisting we give him one more try bc he really wants to be there for us. What can I do to help him and help us????