I have always stated that for Ippo to return to boxing, something pretty... tragic... must happen. Maybe not in the "death" tragic, but something that really shakes him.
Ippo is really worried for his mother, so half assed motivations won't do
Something must happen, either injury, retirement or death, to one of his "close" people. Takamura, Sendo or Miyata.
Alternatively, to the coach. But I don't believe that as Ippo needs the coach.