It is your home. I suspect he isn't paying the rent/mortgage/food expenses. They send 18 year old off to war so he is upset at you abour rehab? He could be in a lot worse places.
You did the right thing. Unless you put him in a burlap sack and in the trunk and then dropped him off at rehab, he went on his own volition and can leave anytime. He is not in lockdown or there against his will. Sounds more like his pride needs to heal
Are you paying for his tuition? If that were the case he needs to thank you. He knows he messed up big time and good for you for sending him a clear message.
He will be ok and get over this. It hurts sure but the benefits of what you felt you had to do outweigh the risks. It is my understanding that pot is now seen as having psychologically addictive properties and it is no longer viewed as 'safe'. Saying it is so is very innaccurate and based on old information.