I will let you know that I have never used drugs. For whatever reason, since high school (so a long time), I have had a boil on my arm a few times, once every few years it seems to pop up. At one point it did get infected and I had to go to the doctor.
So...of course I cannot say if he used or not, but I do want to throw out there that people just get boils sometimes who are not users at all.
Hugs to you!