The crowd at singer Sara Bareilles' concert in Orlando, FL were in for a special treat when 4-year-old cancer patient Joshua Chambers joined her onstage to sing her hit song "Brave" this past weekend.

Joshua has been battling leukemia since February and met Bareilles earlier in the night at a meet-and-greet. She was clearly taken with the little boy and asked him to come onstage to duet with her.

"That song was always about him in his eyes, and in mine," Joshua's mother Mina Chambers told HuffPost.

"[The performance] was our once in a lifetime memory," she added.

Mina also told the website that Joshua is on his last round of chemotherapy before entering a period of maintenance. He'll still be receiving chemo for the next two years or so, but not as aggressively.

Bareilles tweeted out the video of her and Joshua singing with the caption: "Meet Joshua. My angel." Watch the sweet clip below.