Satrin was a year and a half when Sonko, then Nairobi senator, adopted him and his brother Gift in 2014.
On Sunday, the former county boss visited Osinya and fellow initiates at the PCEA Church, Evergreen, where he said they are healing after being circumcised a couple of days ago.
"Today I'm a very proud father," Sonko said.
"They're now being transformed to young adults through our rite of passage from childhood into adulthood."
Sonko took baby Satrin and his brother Gift in his custody after their mother was shot dead during a terrorist attack at Joyland church in Likoni in 2014.
"He and his brother Gift Osinya have been part of our family since then. Welcome to manhood son and always remember that as a family we love you so so much," Sonko said in a statement on Twitter.
"We also interacted and celebrated with other father figures, uncles and male guardians who had also come to visit their sons."
