This what I call the young punisher protocol. He's the son of Winter Soldier and Black Widow. Problem was as soon as they were born Hydra refound Bucky and used the codeword to retake control of him. He kidnapped his son and put widow in the hospital. Hydra used the son as leverage to make him do there bidding while secretly beginning to make him the 2.0 version. As soon as he could walk they injected him with there version of the super soldier program and began to train him at an extremely young age to become an assassin. One day though when he was 13 bucky failed a mission. And as punishment. They cut off his sons arm and replaced if with a metal one just like his to ensure he never failed again. When he was 15 the place thet were hidden at was found out and attacked by the Avengers bucky and Connor were rescued.
He was enlisted into SHIELD but rarely used due to Bucky and Natasha's protest. When he was 16 he had the toned and body build of a 21 year old Olympic athelte.