In reply to Guy Hurst:
Well, there's the obvious one for a young lad; being quick for a bit isn't the same as being able to keep going all day (and it will be all day). So a few days out building both the ability to keep on going and then being confident about it would help. Happily, this can be combined with the second tip, which is . . .
You go with greater confidence if you've been over the ground before. So in order to build that all-day stamina, get him and his mates to walk over what they're going to take on in sections first, combined with other hills. Ambleside to Keswick via Fairfield, Helvellyn and the ridge north of there. Then Grasmere, Helvellyn, Skiddaw to Keswick. Wasdale Head to Keswick via the Scafell group, Gable, Honister and the ridge west of Derwent Water. Shap to the ODG via High Street, Patterdale, Grisedale and the Langdale Pikes. Langdale horseshoe from Grasmere. And others.
Which should keep him out of mischief for a bit. Then plan on organising a bit of support, a point or two where you can be waiting with food and drink, wait for decent weather and the long daylight hours of June and off he goes.
T.