Welcome!
Others here can offer more advice but the one thing I can say is you do not have to finance your son.
Hitting bottom means you detach and he is pretty much on his own to face the consequences of his own actions. If that means he goes through all of his money and cannot pay for school he has to leave. It does not mean you step in and pay. It means if he runs out of money for social expenses in any given month you do not give him more money, regardless of the reason or the crying or the begging. It means you do not hop in with money if he says he is being threatened by his dealer or with expulsion or for bail.
He is in college so therefore he is considered an adult. As an adult, he is the captain of his own ship and he can steer it. If he steers it carefully and keeps his nose clean he won't run it aground. If he decides that drugs/alcohol are part of his life that use will likely escalate and become the center of his existance and he will eventually run his ship aground. The sooner you step back and let him, the sooner he will hit those rocks and seek recovery.
You did not cause it and you cannot control it. You absolutely should not pay for it.