TobyMac has released a new song in memory of his son, Truett Foster McKeehan, who died unexpectedly last October at the age of 21.

The Grammy Award-winning artist said the track, called "21 Years", is an "honest letter" from himself to his son after his sudden death on October 23, 2019.

In the song, TobyMac wrestles with God over Truett's death, saying, "Why would You give and then take him away? Suddenly end, could You not let it fade? What I would give for a couple of days."

Announcing the song on his Instagram account, TobyMac admitted to the challenge of coming to terms with the loss of his eldest child but thanked fans for their continued support.

"The last couple of months have been the hardest I've ever faced," he said.

"Thank you for the love and support.

"Part of my process has always been to write about the things I'm going through, but this went to a whole new level. What started out as getting some of my thoughts and feelings about losing my firstborn son down on paper, ended up a song." 

On Twitter, he added that he loved Truett "with all my heart". 

"Until something in life hits you this hard, you never know how you will handle it," he said. 

He went on to say, though, that he believed God was walking with him through the grief and that He was with his son too. 

"Writing this song felt like an honest confession of the questions, pain, anger, doubt, mercy and promise that describes the journey I'm probably only beginning. The rest is yet to come," he said.

"One thing I know is that I am not alone. God didn't promise us a life of no pain or even tragic death, but He did promise He would never leave us or forsake us.

"And I'm holding dearly to that promise for my son as well as myself," he added.