score:4
The following is what he needs:
Absolutely no other paperwork is needed for the UK - again he almost certainly won't need to see an officer to begin with.
For the US, TIMATIC, the database used by airlines, says:
Minors traveling unaccompanied, or accompanied by one parent or a person other than parent/legal guardian, can prevent possible delays if holding a notarized letter of authorization signed by absent parent(s). For more details refer to https://help.cbp.gov/app/answers/detail/a_id/3643/kw .