Simon wrote his first song in 2000 called Still Searching, which he dedicated it to his dead father. Later, he recorded the song together with his classmate Nick Schilder. Since 2002 he has been making music with Nick and writes songs with or for Jan Smit.
Monique Smit, sister of Jan Smit and also singer, contacted Simon to participate in her first album, called 'Stel je voor' (2008).
Simon is the son of Klaas Keizer and Gerda Keizer-Keijzer. He has one elder sister, Carina. His father died in 1999 from cancer. Since September 2004 he has been in a relationship with Annemarie Hoek, whom he lives with. They became engaged in 2011. The two were married on 13 April 2012 in a private ceremony in Edam, NL. Simon is a great-nephew of BZN-singer Jan Keizer.