Would you, in this situation, say or do anything? (please see details)

My children by complication of custody are back in Australia, they are almost 14. My daughter is coming to live with me in the USA in a few weeks. My son is very popular and has lots of friends, has elected to stay there. I can see my children’s facebooks. I can see that my son is drinking! and smoking weed. He is hanging about with older boys (16–17) Their father, my ex, wants me to think he is a very strict parent, but it is easy to see from the posts on their fb accounts that he has no idea who their friends are, where they are, and what they are doing. My daughter is demanding to come here, and my ex does not want to be sued for custody. I do not want to lose my son’s trust, I haven’t mentioned anything I’ve seen. Should I leave it alone?