It won't be long before his boy will help the girls with their bags.

【翻译】用不了多久，他儿子就会愿意帮那些女孩子拿行李袋了。

【答】在时间、条件、让步等状语从句中，即便两个动作都是将来发生，语法也不允许主从句都用将来时。规定从句使用一般时才代替将来时。

在before his boy will help the girls with their bags 这个时间从句中，will help意思是“愿意帮助”。will是情态动词，表示人的意愿、愿望；不表示将来。又如：

If you'll excuse me for a while, I'll find out for you.  请稍候片刻，我给您查一下。（您若愿意等的话）

If you will keep waiting for a while, I'll help with your enquiry. 请你再等一会儿，我要帮您解答一下。