how can I prove that someone is 100% NOT addicted?

Hello, Im 18 and parents HATE my also 18 year old boyfriend because they found out that he used to smoke marijuana a few years ago. Although he has never been addicted and is DEFINITELY NOT addicted now my parents are absolutely convinced that he is and therefore is dangerous. This does not only mean that I am not allowed to see him ever because if I do they wont pay for my university, it also means that im not allowed to go out ever because they dont feel that they shuld trust me since im with a drug addict. So basically if I want to have a normal life and also see my boyfriend (who I’ve been with for about 3 years) I need to prove to them that their excuse to why he is dangerous is false.

Is there a way to find out FOR SURE if a person is addicted to pot? How wxpensive would it be? How far back would blood analysis be able to detect marijuana? I want them to see that it has been more than a year since he has done it, which naturally means that he is not addicted to the drug.. please if you know a lot about these things do help me find a solution to my problem! if I am able to prove that he doesnt have a problem, it would mean that their main reason and excuse for all they put me through is not correct. thanks!