I agree with GED you have the 'right' to do whatever is BEST for your son and you. Yes, your attorney can get the child custody amended to read that not only must he not be using, he cannot take the child around or have the child around anyone who is using.
J M H O
As a side note, when I read your post and what the GF told your son, I immediately thought of 'listening' bugs, not creepy crawly bugs. With EX being an addict, you just never know.
Anyway, you do what you feel is best for your son.
Love and hugs,