If it was this weekend you were planning on that trip, it’s the BDS, so gondola will be closed to all riders other than those racing.
He’s going to have to be pretty strong to manage the climbs, but there are great flow trails (way better than Blue Doon, which is pretty pish tbh) in Aberdeenshire. Check out Tarland Trails (both the original and much bigger new trails), Birse Forest and then there is a new set of fun trails immediately north of Banchory (only about a 3-5 minute lap, but great fun for a few hours).
Q: Would I drive all that way myself, if I lived down south? Possibly not – it’s quite a drive, but much nicer riding than Nevis (especially for an 8 year old).
If your ‘must have’ is uplift, then it would probably be best to head to Wales.