I would imagine your gut instinct is right – behaviour and personality change plus the fact that it mirrors what he was like before means he is probably doing it again .
The only way you can know for sure is ask him to do a drugs test if he refuses you have your answer anyway and he must face consequences for this . You’ve already said you don’t want him there if he is using or the life that goes with his using so for your own sake you need to put yourself first or you will end up Trapped with him in an addiction cycle