He’ll probably be okay heading up from Threlkeld – it’s a land rover track that turns into singletrack for the final mile and a half / two miles. Nothing too technical and it’s not so bad if you end up pushing a bit. I’d maybe avoid heading towards Mosedale though – it’s very boggy at the moment and he might find it frustrating; it’s not a rock garden but it’s tricky if you’re a newbie.
You could head out to Dash Falls and come back around Skiddaw, although that gives you quite a bit of tarmac (and the Carlisle Road isn’t the nicest to ride down during the day).