So this is what happens to retired sumos... (actually most of them go on to become sports casters, masters of their own 'stable', or owners of restaraunts/bars).
The song he's singing is a famous children's song. The lyrics are basically:
"I can hear the frog's song
kwa kwa kwa kero kero kero kero
I can hear the frog's song
kwa kwa kwa kero keor keoro...etc."