IMO Joe Kennedy was using his sons to rehabilitate his reputation for being a NAZI sympathiser, and I suspect it was Joseph Kennedy not John that was being groomed for a political career.
It's well known that Joe Kennedy was at least a defeatist (when it came to the British), and arguably a Nazi German sympathizer. Please don't ask for citations on basic, easily verifiable facts, it comes off as lazy, flippant, and ignorant.