I would ask for something in return because that means they are obliged to move forwards in order to get what they want. It also means they are forced to do something they don't particularly want to do because it will help them. And then they will have had the help.
Right there sums up your problem. Being clean for a few days at a time isn't doing well. sorry but realistically it isn't.
You've admitted there he doesn't actually want help he just wants someone to wave a magic wand and his life be fixed without any real effort from him.
If he had put the effort in he would have known which module was being tested because he wouldn't have gone into college in a drug addled state.
He's making all the right noises but is expecting everyone else to carry him through it. That is where you are enabling him.
Yes the most traditional type is via cash but this is slightly more subtle yet still with the same results.
Until he gets angry with himself and realises his addiction is what's ruining his chances and he won't succeed until he stops self sabotaging by living in denial. He will not succeed on any programme you put him on.
His brain is not functioning in the same way non addicts are at the moment as the only thing he owes anything to, the only thing he is obliged to right now is the drugs. Being around normal people won't matter his constant partner when he goes to sleep and as soon as he wakes up is drugs. It shrouds him like a bubble and alters his reality and nothing from outside gets in. That is why no help will work because nothing can break in until he has broken out.
The only way he will kick the habit is if he faces it himself. Seeks the help himself. Not says to his family, "I want to get clean" and then you all rush around and sort it for him. A test of his resolve that his will to do this is finally stronger than the drugs is if he sorts out the logistics himself.
The feeling of being obliged to someone else is just fuelling his anger.
You can't stop him ruining his life if that is what he is intent on doing. You can't save him for your MIL. As again that is adding fuel to his rage because its not truly 100% for him and he knows that. Any attempt at coercion will add to his rage and any half hearted attempts to say the right things without the commitment will have the same result.
Nothing you do will work until he really gets it himself and you are banging your head against a brick wall until that day.
Unfortunately the talk about hitting bottom is true. Every addict has that one appalling moment that shocks them into no holds barred self examination and leaves them horrified at what they have become. Only then do they commit to seeking help.
Some are lucky and it takes one abhorrent moment, others have to face a series of ever lower events. Here your BIL has never really had to face any shocking consequences of his addiction. Being kicked off a couple of courses isn't really going to sink in. Withdrawing until he has faced up to it and hit his bottom is all you can do. The more you extend your hand, the slower and more agonising his descent. You are just propping him up with no real foundations and thus making the fall even harder to take when it happens.
The Al anon suggestions aren't a poke at well you and your H must be alcoholics. They run courses on understanding the mechanism of addiction you get to hear things from an addicts point of view, albeit alcohol but the mechanism is the same. They can give you tips on what to do to be ready to help without enabling. Tips on what they will do or say when they are finally ready to quit. They will teach you coping mechanisms and how to show compassion without getting drawn back in to the addiction drama. They will teach you signs of a relapse and how to react to this without helping push it back to full blown addiction. They will also teach you how how to cope with the fallout of addiction as a family. How to handle the feelings this raises within it.
I wish you well.