I’d say go for a 14, depending on his size of course. My eldest went from a Rothan to a Cnoc 14, he was ready for pedals but the 14 was very slightly too big for him so no way he’d have fitted a 16. I ended up using a different seatpost and an upside down clamp to bring the saddle another 15mm or so lower which was just right for him.
Because he could reach the ground with confidence he was flying very quickly, probably 20-30 mins until he was pedaling unaided and another 30 mins perhaps before he could get himself started.
He’ll probably be getting a 16 in the new year sometime but only because his brother will be ready to go to pedals and will get the 14.
Not sure I’d agree with the above from scuttler and jonathon, IMO they need to touch the ground comfortably to be able to get going properly, but that is just my opinion.