I'm assuming you mean that the kid that wasn't vaccinated wouldn't have antibodies in his system? But how do you tie that to "This is definitely the kid that gave the measles to my child".

Could have been that kid in his class that is unvaccinated. It could have been a kid he hung out with on the playground, or a kid he walked past in a mall.

There's no way to prove beyond reasonable doubt that just because the kid in his class wasn't vaxxed, that he was necessarily the specific vector for your child to get measles. It's impossible. To many variables.