Usher explains why he missed the One Love Manchester concert

— -- Usher's absence from Sunday's One Love Manchester concert had some fans scratching their heads, but the singer had a good reason for not performing.

In a statement posted on Instagram Monday, Usher explained that he was spending time with his son.

Usher's 9-year-old son, Cinco, has diabetes, and yesterday was his first day at Camp Kudzu, a summer camp in Atlanta for children with the disease.

"This was an important day for him and for myself as a proud father," he wrote, adding that he "would have loved to be there." "Stay strong UK."

Singer Ariana Grande, 23, organized the One Love Manchester concert after a suicide bomber killed 22 and wounded more than 100 at her May 22 show in Manchester, England. Stars including Katy Perry, Justin Bieber, Coldplay, Miley Cyrus and Pharrell Williams all performed to raise money for the victims of the attack and their families.

According to a tweet from the British Red Cross, $13 million has been donated to the Manchester Emergency Fund, including $3.49 million raised during the concert.