我觉得填 later 比较合适，later 在此表示“稍后”。又如：

Save your strength for the hard work you’ll have to do later. 留着点儿劲儿，你一会儿还得干重活儿呢。

We’ll have to eat later, I’m collecting Grandma from the hospital this evening. 我们得晚点吃饭了，今晚我要去医院接外婆。

如果用 then，句子应该会这样安排：

He washed all the dishes (and) then his mother wouldn't have to do it.