I have known a lot of people who use drugs but, I don't know of anyone who can use cocaine or heroin just occasionally. As for being able to occasionally party that is a lie they tell themselves to get started again.
I am sure it is confusing. I am confused just trying to write a reply to this thread. I used to use meth amp. I have thought about how I felt while high many times since getting clean. I don't feel like I am missing anything and I have never been close to trying to purchase some.
Hopefully as time goes on thoughts of using will fade for your son and he won't envy people who can use occasionally or try to plot it out for himself.