How about this one http://www.mangago.me/read-manga/the_4th_guard/
Tight Rope is a bit like that. It's a bout a yakuza boss' son and his best friend, the yakuza son is stronger but he can't beat his best friend because...
http://www.mangago.me/read-manga/hizamazu_ai_ni_kogareru/
http://www.mangago.me/read-manga/pet_ja_nai/
http://www.mangago.me/read-manga/kimi_wa_pochi_da_daigakuseihen/
http://www.mangago.me/read-manga/tsuki_ni_hoero/
http://www.mangago.me/read-manga/joouheika_no_oniwaban/
http://www.mangago.me/read-manga/ijippari_love_panic/ ch3
http://www.mangago.me/read-manga/koi_no_amai_toko/
Are there any yaoi manga about bodyguard? I Think the story about somewhere like the boss end up being stronger than bodyguard but he hide it from him something like that. If there are any recommendation please let me know :3