Lots of good info here.
I think it is important to remember that the first few days are tough mentally. Like a black hole trying to suck you back in. But the longer a person is away from a drug the less pull it has and other areas of life take over. If it did not get easier then no one would quit anything ever. So the obsession will go away. But the obsession is what the pull is the first few days.
I had a cocaine addiction in the 80's but opiates were my main downfall. But for a few months I kept going back to cocaine. It didn't help my family was running a service station off of I-10 in Houston. My dealer was the rodeo champ of that time in the area. He would come in in his cadillac, tell me no money needed right now and that it is fire right up from SA. So yeah, it was tough. But I got away.
Cocaine deals with the reward system, so maybe your son can take a road trip or vacation, anything he likes to do AFTER taking the bull by the horns. It is also important to know in time he will feel better. It is just that we are complex human beings and all the issues the drugs were pushing aside come back up. That is where the support system helps.
So yeah, maybe a trip to Hawaii after a month of abstinence.

Something to look forward too. But under no circumstances should any form of enabling happen. It is not negotiable. That is the vibe where it should start. As his mother look him confidently him in they eyes, understand it is tough, but has to be done. No excuses. He is not the first crack addict and certainly will not be the last. No enabling. He is 20, young, strong and can wrestle this bull to the ground. I say that because my mother did not take any crap from me when I had to live with her at times. She set the ground rules and I can say years later it helped.