Another mom here. My kids couldn't manage to go to college, but my son did enroll for nearly a year.
How are his first semester grades? Do you have any access to those (probably not - we can PAY for the college, but the stupid rules say "privacy" won't allow us to know what our kids are really doing... grrrrr).
I think $100/week is pretty high, considering he is getting all meals. Why not tell him you will be sending $100/month and it is his choice how he spends that. The amount can go up, if his grades are all good.
Then hold firm... for me, that meant NOT answering the phone when they called. It was far easier for me to be strong if I did not have to listen to the "quacking".
My daughter's drug of choice is meth, and she couldn't even make it out of high school. But she also never needed any money from us - she was able to get her drugs through friends and by being a "runner" for the dealer. My son got his drugs by stealing from trucking companies and selling the scrap materials to a recycler. He also worked as a runner for the dealer.
In both cases, their appearance changed drastically. Daughter's make up got darker and darker... she dyed her beautiful red hair a dingy, icky brown. Son stopped showering and brushing his teeth and his clothes and car were always dirty. Daughter (the meth addict) got really bad skin... and when she got real bad, she started having sores in weird spots - like her arms and upper legs. The picking was due to nerve damage that makes them itch and pick (they call it meth bugs).
Neither of my kids EVER ever gave me "permission" to set a boundary, so I expect your son will balk at the reduction in funding - no matter what his state. But if you believe that $100 a month is reasonable, then his squawking doesn't change that.
I also started looking at the cell bills... egad. Both kids "rented" their cell phones to drug dealers... and the number of calls to folks I didn't know was incredible. They both also "lost" phones that needed to be replaced. And they both started wrecking cars. Son wrecked two and daughter wrecked five in one year.
I honestly thought daughter needed more instruction in driving and paid to have her retake some driving at the driving school that had graduated her... I found out from her drug counselor later that meth impairs the peripheral vision. Jeeze.
Anyway.... it is a rough road, being the mom of addicts. Alanon was the biggest help to me... and my home group is made up of LOTS of double winners. I think you would find it very helpful. I know, I did.
(((hugs)))