The worst thing you could do to your son is enable him. Because allowing him to stay when he has no intention of keeping your rules (rules I agree with totally) means that you are letting him win in your relationship, at the loss of his character.
I say this, because I went through this. I was very defiant towards my parents. I broke one of their rules, although not for the reason they thought I was. They threw me out at age 17, in the middle of grade 12. I put myself through hell after that, but I did get stronger and stronger. I learned how to fend for myself, and put my life back together. My brother, on the other hand, was totally enabled by my parents. He used their car, blamed my sister when he crashed it into the garage, they excused him when he molested my sister, because "boys will be boys" and he managed to flunk out of every training course my dad got him into and he was not dumb.
Meanwhile, I got a science degree and a teaching degree. I got married, and had 4 children. Bought our own houses, built a house, and worked for a living. My brother did not have to work, because he could just stay in my parents basement. And stay, and stay, and stay!
My brother is now 60 and living in the basement. No job, no wife (he had a nice girlfriend once (a Christian) and my father forced my brother to dump her, because my father didn't like her mom. Eventually, the mother got remarried, leaving the girl behind. She committed suicide a year or so after that. And of course, no children, no interests and no life.
My sister and I have tried for 6 years to get our mom and dad into assisted living, and then my mom when my father passed away. But, my mom can't move, because this marvellously manipulative son threatens to commit suicide every time we try to get him out of the basement.
Now your son may not go through all that. But, you are hurting him for sure, allowing him to have his cake and eat it too. You may think you are doing him a favour, allowing him to flaunt your rules, three days in a row. But you are hurting him.
Why do parents think enabling their kids is the right way to do things? Our society is just so mixed up about the real meaning of parenting! In my opinion, three strikes and he is out. Well, with a girl he has known a week - probably one strike sealed that deal for me.
PS. We raised 4 children. Not once did they try anything like that. In fact, our oldest son had been married 8 or 9 years, and he was so excited he got to stay with his wife in our house. Our children all have good careers, their own homes, and, of course, our grandchildren are wonderful! We gave our kids a lots, in terms of taking them to sports activities, music lessons, camping in the summer, and lego. Yep, lots of lego! LOL. But they have not disappointed us. We are so blessed!