。
Something must have happened to him on the way here.
Literal
Come midway [at-で] he [possessive-の] body [to-に] something [subject-が] happened [must-be-にちがいない].
~途中 ('on the way / midway') is a noun meaning 'in the middle of a journey.' 身 ('body / person / self') here in the idiomatic 身に起こる ('happen to a person') — the に marks the person the event happens to, 身 being a slightly metaphorical 'on one's person.' にちがいない marks confident inference.