7-Year-Old Cancer Patient -- Duet With Favorite Singer Rachel Platten (VIDEO)

7-year-old Jeremiah Succar is a huge fan of Rachel Platten ... and especially her hit song -- "Fight Song," which has become a driving force for him in the hospital.

His father explained, "he used to sing it when he got a lot of headaches, but now he sings it in the morning, before bed and during shots he has to get.”

Jeremiah has stage-four atypical rhabdoid teratoid -- a fast growing brain and spinal cord tumor.

Platten went to Children’s Hospital Los Angeles to surprise Jeremiah after finding out he was a fan on social media ... but he was the one that surprised her when he sang the song along with her.

Fight on Jeremiah ... fight on.