I do not attempt to justify it.
If treated as Geneva Convention POWs they would be incarcerated until the end of hostilities.
That could be a vey long sentence.
Al Q, as you say, abused this boy, using him as a fighter.
As a result of that abuse, US forces were put in a position of having to deal with him.
If released he would go back to killing them.
So what to do?
(The Russians would have killed him. They had no Guantanamo equivalent.)