He was excited because he was scheduled to leave for America the next morning.
Literal
Next-morning America [to-へ] depart [ことになっていた] (it-was-arranged) [because-ので], he [topic-は] was-excited.
ことになっていた (past of ことになっている) marks the established past arrangement. ので gives the causal connection. 興奮する ('to be excited / agitated') + ~ている makes it an ongoing state — 'was in an excited state.'