You did say ex correct? What he does with his time on the weekends should have no place that matters in your life. Unless you share custody of children and his drug use is a factor.
I think the better question you might focus on is, why does it even matter to you what he does, he's an ex and you are struggling to move on.
The more free rent you allow him to occupy in your mind the less space you have for you and your future happiness.