It's ok - he hasn't killed it - he's pruned it a bit wrong, but it'll survive
It may not grow any long shoots this year, it might just have a couple of leaves or so, but next year it'll grow some longer shoots again - and it'll have better roots by then and should grow just fine - and by then your son will be a year older and know a bit more about gardening.
Why not help him sow some nasturtium seeds in the patch in front of the grape vine - they enjoy being in the sunshine and their roots won't damage the vine at all - and the whole plant is edible so your son can eat it if he wants to and he'll come to no harm. I like the leaves in sandwiches with a little salami - they're a little bit hot like watercress, and the flowers can go in a salad, and you can pickle the seeds and use them as capers.
When my daughter was a little girl she grew some every year - one year they were eaten by caterpillars, but instead of being sad, she said she had a butterfly farm