His dad isn't an asshole, companies are very abusive to workers in Japan making them work long hours for no extra pay, etc. Also it could be a bs reason, maybe his dad just wanted to ensure his son got exercise and could defend himself. Knowing you can beat someone up who's bullying you, even if you don't, helps mentally alot. So if at work his boss was abusive he could always think "yeah but I could crush your wimpy ass like nothing, you're lucky I want this job", it would help alot with Japanese workplace abuse.
Also, this guy gonna be turned into a human sex slave once the rest of the female society finds out about him. Just a stud for breeding. Looking forward to seeing how this society currently reproduces.