About a Boy

Will Freeman[2] (Hugh Grant) lives a serene and luxurious lifestyle devoid of responsibility in London thanks to substantial royalties left to him from a successful Christmas song composed by his father. Will begins attending a support group, called SPAT (Single Parents Alone Together), for single parents as a way to meet women and as part of his ploy, invents a two-year-old son named Ned. His plan succeeds and he meets Suzie (Victoria Smurfit). Will brings Suzie on a picnic where he meets Marcus (Nicholas Hoult), the 12-year-old son of Suzie's friend, Fiona (Toni Collette). Will gains Marcus' interest and trust after he lies to a park ranger to cover up for Marcus killing a duck by throwing his mother's concrete loaf at it. Afterward, when Will and Suzie take Marcus home, they find Fiona in the living room, overdosed on pills in a suicide attempt.
Marcus attempts to fix Will up with his mother in order to cheer her up, but the plan fails after a single date. Instead, Marcus becomes close to Will after blackmailing him with the knowledge that "Ned" doesn't exist, and begins to treat him as a surrogate big brother. Marcus' influence leads Will to mature and he seeks out a relationship with Rachel (Rachel Weisz), a self-assured career woman, bonding over their experiences raising teenaged sons, though Will neglects to explain his relationship to Marcus. Marcus, in turn, becomes infatuated with Ellie (Natalia Tena) but gives up his romantic interest in favour of a close platonic friendship. Will, realizing that he desires true intimacy with Rachel, decides to be honest with her about his relationship with Marcus, but this backfires and their relationship ends.
One day, Marcus comes home from school to find his mother crying in the living room. Marcus attempts to unburden himself to Will, but Will is withdrawn following his break-up. Marcus decides to sing at a school talent show in order to make his mother happy. Will attempts to return to his previous lifestyle, but finds it unfulfilling and decides to help Marcus. He crashes a meeting of the single parents support group to find Fiona and beg her not to commit suicide. She assures him that she has no plans to do so in the immediate future and reveals that Marcus has decided to sing at the school show that day.
Will realizes this will be a huge embarrassment for Marcus and rushes to the school to stop him, but Marcus is steadfast in his decision to perform, believing it will be the only thing that will make his mother happy. Marcus steps on stage and sings his mother's favourite song - "Killing Me Softly with His Song", the student body starts to taunt him. Suddenly, Will appears onstage with a guitar to accompany Marcus for the rest of the song, earning themselves a modest applause. Will performs an unnecessary solo immediately afterward, turning himself into the butt of the joke and rescuing Marcus from humiliation and even more social suicide.
The following Christmas, Will has gotten back together with Rachel and hosts a celebration at his place with his new extended family. The idea of Will marrying Rachel is brought up and Marcus seems unenthusiastic. But Marcus reveals in voiceover that he's not against Will and Rachel marrying, merely that he believes that couples don't work on their own and that everyone needs an extended support system like he now has, concluding "No man is an island."[3]

Question: Who joins Marcus during the performance?
Answer: Will