man I have tried so much to use that perk. I have a ton of mexican co-workers, one of which went to Guadalajara with me last month, and another is 15ft from my desk. I will ask them questions frequently but it's never really enough to "just go for it" and converse in espanol lol. I do better when I'm forced into it Mexico and spanish is my only option. my ears start to pick it up way better when I'm in the country surrounded by it all day.
My son, being 14, is not yet comfortable in a public squat rack surrounded by yoga pants everywhere when attempting foreign ideas so we haven't really pushed it that far. in general I'd say no, his biggest fail is knees caving in and heels coming up, but I'm already seeing him fix that big time. I'd have to see your sons squat, generally speaking relaxing the lumbar is either from just not being aware of it or simply squatting too deep. I agree with Kleen, a buttwink in and of itself isn't an injury waiting to happen, where I have seen sorness (still not really an injury though) is guys relaxing their lower lumbar area deep in the squat like on a pause squat for a number of reps.
Love the idea of goblet squats as a squat substitute for now as well. good on you!
I'm trying to imagine a scenario where not getting his butt out is causing the butt wink though.
Generally speaking the things I'm watching first are knees, heels, and good mornings. From there I'm looking at bar path. even though I've gone away from high bar, I think it's fine but a common fail I see in newbs is a higher bar position with a low bar squat so what ends up happening is the barbell is more over their toes than mid foot at the bottom (= sore lower back).
the more I think it through it sounds like you're describing your son as maybe having a low bar position with more of a higher bar style squat?