You don't even need to let him whip the retainers(Since banning Whip increases Retainers molare significantly).
Basically:
1) Be polite with him in the chapter 3 after winning the race. Be polite with everyone, to be honest. Politeness and Patience are two virtues you must carry with yourself regardless.
2) When Scholar Taio is interrogating you don't keep in silence. This will make Bo Lo be tortured too and that reduces your reputation with him dramatically. So either tell the truth(Recommendable. Always be honest) or successfully lie to Taio.
This will already be enough to make Bo Lo fight in the Suon event.
3) You can ban Whip normally but the next step is mandatory on that case:
4) Train him in the 3-day trip until Nanhou(Post Harehama). This will increase his combat prowess and also your reputation with him so he will help the twins in the Rabbit trial normally and also ask to train with you in the Mist Chamber.