H4 - Student GC priority date

[svnr]

Hi,

My son is a high school senior and planning to join university for undergraduate. I am in I-140(EB2) state and priority date is in 2016.As per current trends my son will not get GC before 21 years. As per my knowledge he should apply for H1 after his graduation to work. My question is does he has to start his GC after he gets job or his priority date would be 2016 along with me? 

Thanks,

svnr