Are you sure you're thinking KG here, not lbs? 120KG for a 12yo is pretty hefty, even if above 2 meters tall!
I personally don't like leather chairs - they don't last as long nor are as comfy as fabric ones.
@crmaris - drop the personal comments about weight in reviews please. People can be above 120KG easy, specially if tall and / or very sporty (lifting weights, etc.) Also those comments have no place in a chair review.
On another note, the seat has only 35cm before it's getting wider to 56cm? The flat version of seat that is below 54cm I already find uncomfy small when sitting for a longer time as I am also male and like to cross at least 1 of my legs below the other if unable to sit completely cross-legged. I currently have a chair I also paid €300+ for, but out of a real office chair shop with 58cm seating area where the seat is flat and for me that is just about wide enough for comfort, but then again I had to go to a "larger" chair with better weight carrying capacity (I have a wife who like to keep me company as well and sometimes even sits on my lap
) and thus 120KG max I find rather miserable.
As for the €550 price-tag, way overpriced for what I see.