Depends how up for it he is.
For trail centres around that age mine liked FoD, Timberland, and Cwm Carn (climb on road, or better - take uplift).
One of mine at that age wrote a story for school entitled 'Uplift Day at Mount Mojo' in fact
A mates done Penhydd with his kids, about that age, but it took a long time.
I think Bechfa blue would be a similarly good long ride.
Last year in the alps I saw a guy towing his kid up a long climb using a long luggage strap. Kid was pedalling a bit but its not as full on for them. Wish I had thought of that.