I did chores as a kid and teenager, and I also helped my dad a lot with DIY projects. Including building an extension on the house, putting up a shed, re-wiring the house, and replacing a lot of the pipes in the house.
If my dad hadn't had such a nasty temper and expected me to be able to do things that were unreasonable (like expecting a 12 year old girl to be able to help lift and carry extremely heavy things that two full grown men would have trouble with), it would've been enjoyable and I would have gotten a lot of value out of the experience. Learning to do these tasks will help the son in the future. As long as the grandparents didn't task him with things he wasn't physically capable of doing, or couldn't do without risking serious injury, and as long as he was allowed to take breaks (meals, bathroom, etc, which I wasn't allowed to do) then I see no issue with it. One day the tween will be a man who will probably own a house that will need repairs done to it. Having a basic knowledge of how electrical wiring and plumbing works could save him a lot of money.
I do think the grandparents should have cleared it with the parents first, though.