With your brother and sister story strength that can't be explained usually goes one of 3 ways that you have to be careful with. It's usually either alien, which is unlikely in this story so far, genetic manipulation/mutation, or cybernetics. If you want to go one of these routes approach it carefully. Reveal little things about the father or the son and space out those reveals. Or if he's just unnaturally strong like because of something that is a bit different in his body then just eventually have the father explain it. Just make sure you know how you want it to go and commit to it. If done incorrectly it could mess up the story.
