In reply to GoWest:
I'd have thought he'd be fine, at least in half decent weather. My older son is 8, and having got a new bike for Christmas, is cycling the 16 or 17 mile round trip along part of the C2C to his grandparents house apparently without noticing. I'm sure he could easily do double that in a day.
Given your son knows he can cycle 40 miles, seems a fairly reasonable challenge