If you want the grandson to be able to do much more of the work – and want the price to be lower, and the results quicker – you might like to think of starting him off the way we all started in the 1950s and 1960s – with a small balsa 'Junior' kit.
There aren't many (any?) available from the shops, but extensive plans and support can be got from the two sites below, which enable you to recreate these old kits for free:
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If Grandfather is a modeller he can easily cut out the parts, and balsa is a sufficiently forgiving material for a novice to be able to work on their own rather than watching as an adult builds 'their' boat.
Here are some youtube videos:
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