Having worked with heroin users, I conclude that only those who are ready will change.
Your brother in law is partly ready and you commend him for this, and rightly so. Tell him you believe he can do it and so on.
But I agree with the others. If you continually bail him out, he won't learn. The difficulty is that if you stop, he will do what he wants and things may get worse until he realises he is on his own, with only professional support and believe me they won't pay for him.
The other issue is that the dice he is rolling are very dangerous and he risks overdosing all the time. The stakes are therefore high. But you can't stop this. There is nothing you can do make this better without his involvement and while eh thinks you will do for him, he won't learn his own coping skills.
Also could he be looking to do a tefl course to go somewhere where smack is cheaper??
The thing users always say is that the issue is outside of themselves. Your brother in law is saying that, if only he could get away from the estate, he would stop using. This is wrong. Temptation will always be around, it is him that has to learn how to deal with it, not for it to go away.
That is the biggest tell tale sign to me that your b I l is not serious about stopping. He has not accepted that he is the problem yet.