There is plenty of evidence out there showing that homework doesn't improve educational outcomes. Often it does the reverse. Kids that need to improve their skill and understandings don't do homework and end up resenting it and the rest of their learning because of the stress and strain it puts on them and their families. Your sons head has made a decision based on sound research and data.
Incidentally though the Head can't ban homework, if you want to set your son some then you are well within your rights to do so.
What evidence exactly? My friend's son is privately educated and has had formal homework since he was 6. He's a good year ahead of the kids in the same year at our LEA school and it's been ranked "outstanding". The way that's written sounds like it's progressive nonsense written without a care for the brighter students that need pushing. Quite apart from the fact that in the world of work you will often need to do overtime, take work home and be brutally assessed I think post #1 in this thread shows what happens when educational research meets national curriculum.