The relationship sequence:
You would be his Great Uncle and he would be your Great Nephew.
3rd cousin
You are their great aunt/uncle.
He would be his great great great uncle
They would be great nieces or great nephews.
Great nephew
Second cousins once removed.
They are your great nephews and/or great nieces. Alternately, some people say grand-nephews, grand-nieces.
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
Your nephew's son is your grand nephew or, some would say, your great nephew. You are his great aunt or great uncle.
Your nephew's son is your great nephew.
Great aunty