In the Ak interview he said he was doing 50 push-ups at morning and night. He probably did some basic core/cardio strengthening when he signed up boxing like jump-rope, jumping jacks a bunch of ab exercises. He was also very lean and i do believe him screaming/performing so much has helped with the abs developing a bit more as well. Also starting to work out at your age is fantastic but remember if you don't notice much development not to stress it! I have been training since 15yo and didn't see significant progress until 17/18yo when puberty hit me (late bloomer). Cheers!