He’s a child, but he’s also a major character. We won’t be able to use his physical qualities to help determine his Enneagram — he’s not mature enough for that. The costume designer has made strong choices, though, that give him a definite physical presence.
For instance, awkwardness. The shoes are in the script. Marcus wears a brown loafer, something unathletic (as well as uncool). The trainers are his for only a brief time, basically showing that they aren’t a physical match for his character (as well as continuing the bullying storyline).
Not a Body Type. Not a Head Type, either. He’s clearly a Heart. Community, connection, social gathering — these are the theme. Marcus represents all the arguments for these things, going directly against (and ultimately persuading) Will and his lifestyle. Marcus wins, and he does so by repeatedly throwing himself into a difficult social milieu. Very Heart Type.
He’s not a Three. Can Threes even be bullied? And he’s much too unstylish to be a Four. That rainbow sweater! Oops, jumper, as they say across the pond.
He must be a Two, which makes sense with his generous nature. Instead of collecting gadgets or gewgaws, Marcus collects people. Friends and family.