I have met a four year old on the summit of Ben Lawers. (Which is over 1200m, or - in old money - a tad short of 4,000 feet.) I later hitched a lift with her dad and had a long chat. She got up and down under her own steam. The previous day she had done Scheihalion. In the car she kept pointing and hills and saying "I want to do that one tomorrow!" And then fell sound asleep.
So you might be surprised what your lad is capable of, if properly motivated.
I took my 5 year old son and 6 year old daughter up Ben Nevis many years ago, both of them made it to the summit and back to Glen Nevis under their own steam and without a complaint. It must have worked because now in their late teens they are both hardy outdoor types and have tried and/or taken up a range of outdoor pursuits (climbing and caving to name but a couple) I wouldn't have considered when I was their age.
Getting them interested at a very young age seems to work ...
Yr Eifl on the Llyn Peninsula, a stone's throw from Snowdonia. 1-1.5 hrs to the top from the car park, plenty of rocky bits for kids to scramble over and the 360 degree view from the top is stupendous!
Moel Fammau (1818 ft) is a very easy mountain to do, park at the highest point of the pass (car park) on the minor B- road, (off the A494) between Mold and Ruthin, from there a broad track leads gradually to the final pull to the summit, should only take you about 1 hr with a little one. good views from this highest point of the Clywdian range, west to Snowdonia, north to the North Wales coast and East to merseyside and Cheshire, This was my Daughters 1st mountain, aged aroud 5 years old
