、。
If it happens to rain, he probably won't go.
Literal
By-chance rain [subject-が] falls-[conditional-ば], he [topic-は] won't-go [probably-だろう].
彼はいかない writes the negative 行かない in hiragana — a stylistic choice for casual or informal contexts. Combined with だろう, this is a plain (non-polite) future prediction: 'he probably won't go.' The 万一+~ば opening hedges the rain itself as an unlikely-but-prepared-for contingency.