No, it doesn’t negate everything he has ever done. Hopefully, it doesn’t negate everything he hopes to do in his life. That will be up to him in many ways.
He’s about to find that the path before him won’t be as smooth now, because of others who hope to profit from his mistakes.
yes, a lot of very judgmental people are going to make it pretty hard for him now. If he repents truly and helps people to get off of pot it will turn into a blessing. Most young people try the stuff...not a good thing but understandable. I was addicted to it for about a year when I was very young.