Hi Thorlz, just seen this thread and thought I’d throw my thoughts in as I was tossed on the horns of a similar as dilemma recently.
Firstly my eldest daughter measures up pretty much the same as your son and has long since got too big for her cnoc 16. I really should have got her a beinn 20 large months ago but never got round to it as she was happy racing about on the cnoc.
It was her sixth birthday this week so after much deliberation I ordered her a beinn 24 rather than a 20 large which would have been the best fit for her right now. I adjusted the 24 down as small as possible and she took to it no problems. Is it a little bit big? Yes, slightly, but she can ride it confidently and safely and it will last her beyond next spring when a beinn 20 large possibly wouldn’t.
I think you have made the right decision…. we are near Leek if you are not sure.