From Mangahelpers:

Street thug Ryuji has never lost a fist fight and his name was famous throughout the gangs. But he was no match for a gun and he was killed at the age of 35. Through some miracle, his soul is transferred to a 16 year old boy’s body and gets a second chance at life. He soon learns that his new body is actually his son who he abandoned when he left his then wife Ryouko for another woman. Reborn as Sawatari Yuusaku, and living with his ex-wife, turned mom, he tries to restart life as a high school student. Everyone can’t believe that Yuusaku has transformed from a kid that was bullied into the strongest brawler in the city. Ryuji/Yuusaku tries to right the wrongs that he made in his previous life. [vyc]