Rooney who is Manchester United and England’s highest goalscorer of all time, uploaded photos on Instagram on Friday, December 18, showing Kai holding up a shirt with No10 on the back – the same number his dad wore during a glittering career at Old Trafford.
Former England captain Rooney wrote on Instagram: “Proud day. Kai signing for Man Utd.
“Keep up the hard work son.”
Writing on her Instagram page, Wayne’s wife, Coleen added: “Special night….. congratulations Kai. I love you and I am so proud of you.
“Keep trying your best.”
Rooney starred for Manchester United for 13 years between 2004 and 2017, becoming club captain in 2014.
He is the club’s record scorer having scored 253 goals in 559 appearances.
Rooney, now 35, also won 12 major trophies for the Red Devils, including five Premier League titles and the 2008 Champions League.