How much of this is put down to puberty?I'd say at least 1/2. I personally put on 20kg over 2 years just filling out my body without even lifting weights. (Note, I said filled out, not fattened up) A 16-17yo kid grows like a weed regardless of training or diet.Max has gained 30kg in 30 months.
If Max started lifting at 25yo, once he was a fully grown adult, I doubt he'd have managed 30kg in 30 months.