This is an unfortunate situation. I understand your desire to help him and your heart is in the right place. But too often I see kids struggling with the weight and shaft flex of those cut down clubs.
The best thing you can do for him is limit the clubs you get him to just four. A 3 wood, 7 iron (or 6), Pitching Wedge, and putter. Make sure they are light and flexible. This will go a lot further toward his development than a full set of cut down clubs.
If you really only have that one set to work with then there are some things you can do.
You can machine down a lot of the weight of the clubhead. Get it light for him. Then get them reshafted with kids or weak ladies shaft. Just cutting them down does not make them light enough, nor flexible enough.
Again, I know you're trying to do good... but just cutting adult clubs is often much more harmful to their development than people realize.