Hello Concerned Mom72, welcome to Psych Central.

I don't know what these zantrax slim stick powders are but you have every right to be concerned. You are the parent and you need to do what you think is right. I know a 16 year old can get very upset when parent's interfere but as you say this could be serious particularly as you say that he has lost so much weight recently. Do what you feel you need to do, you are looking after your child's health and wellbeing.