I assume (rightly or wrongly) that he wants to improve his physique. As a mother, you are doing the right thing, researching before talking. Getting facts.
We all want this kid too stop what he's doing, but he's surely thinking 'i'm not stopping, i'm growing and liking the results' so how can we motivate him to stop?
Give him the hard facts:
Using Winstrol will shut down his own testosterone production. at 16 years old, in puberty, his Testosterone is hugely important to his development.
If he comes off, and sorts himself out with a good P C T, then with his naturally high Testosterone levels, coupled with a high intake of protein, he would see better gains than using Winstrol.
Once his body has matured, and after a few years in the gym, he is struggling to overcome his natural limits, then that is the ONLY time to consider pharmaceutical assistance.
Winstrol also makes a lot of people depressed, and at 16, with hormones all over the shop, that is not a good place to be.
I feel this is a difficult situation for you to resolve, as you need to keep him motivated, help him understand what he is doing, without making him more depressed. I would recommend sorting out a course of P C T for him, and help him realise that this will bring his bodies own steroid (testosterone) back up to pubescent high levels.
Good luck!