A mother has captured the moment her son asked her husband to officially adopt him in a heart-warming video.

Kalani Watson wanted to make his 10th birthday extra special – so he popped the question to his stepdad Brandon Williamson.

At a party in Los Angeles on Sunday, Kalani read Brandon a letter about their relationship.

"Dad, I know today is supposed to be all about me, but today is really about us," he reads. "Brandon is what I met you as seven years ago. I'm so glad that has changed to Dad. I've learned so much from you....

"I know I met you when I was two, but you have been there for everything. All my school programmes, and parent teacher conferences, all my sports, every time I'm sick... That makes you my real Dad, right."

"I have three questions for you," Kalani says. "Do you love me?

"You know that I do," Brandon answers.

The boy the asks: "Do you love mum?"

"You know that," Brandon says again.

"Can you live without us?"

Choking up, Kalani's stepdad shakes his head. Kalani then finishes his letter with the most important question.

"So let's stop playing and make it official. Brandon Craig Williamson, will you do me the biggest favour in the world? Will you adopt me?" his letter concluded. He then hands over a box containing adoption papers.

The video was filmed by Kalani's mother Porsche and posted to Facebook, where it has been viewed more than 1.2 million times.

"My son asked my husband if he can adopt him!!!," Porsche exclaimed in the caption. "I swear this was one of the best moments since we've been together. Thanks for always loving him unconditionally and treating him as yours from the beginning!! Us against the world!!! Forever and always!! Let's make it official!!!"

Brandon Williamson
Stepfather Brandon Williamson wells up Storyful/Porsche Williamson/Facebook