MK677 works differently, should have no issues stopping. PCT isn’t for the liver, it’s to restore functioning of the HPTA. The LGD certainly may have affected this, but in such a short timeframe, he should bounce back fine.
All that said, it kind of sounds like he’s going to be taking this stuff whether you’re okay with it or not. It wouldn’t be a bad idea to have a talk with him and work out a game plan for what you guys think would be the safest way to use it. I’d recommend teenagers staying away from anabolics, but if he’s going to be using anyway, it would be best to try and keep him as safe as possible.