We’ll have to go without him （）he comes before our flight depart

We’ll have to go without him （）he comes before our flight departs.

if 

providing 

unless 

on condition that

除非他在我们起飞前离开，否则我们只好不带他走。unless除非，符合句意。 if如果； providing以···为条件； on condition that若是。