Hopefully a MOD will be along to move this as it has nothing to do with BA, even if he is flying BA.

There are no particular legal requirements. But, the following are considerations:

1. CBP's concern is that he will not leave the country and will become a "burden" to the US as a minor. Thus, proof that he in school, expects to return to school (or has a job) and has family support in Norway, are all important.
2. He should have some documentation that he will be supported or has the means to support himself in the US.
3. He should have properly notarized English-language documents authorizng the relatives to approve medical treatment (not a CBP requirement, but common sense)
4. He should have properly notarized statements from you as his parents acknowledging his overseas travel alone as a minor (not a CBP direct requirement, but a human trafficking concern).

Finally, he should have mobile numbers for the relatives and someone ought to be available to speak with CBP (by phone) if there are any questions.