Your nephew's son is your great nephew.
3rd cousin
You would be his Great Uncle and he would be your Great Nephew.
Second cousins once removed.
You are their great aunt/uncle.
Nephews and nieces
2nd uncle I believe
your nephews
The sons of your son-in-law's siblings are his nephews. You are not related to them.However, your son-in-law is married to your daughter. The sons of her siblings are also your son-in-law's nephews. They are your grandsons.
Great nephew
Your uncle-by-marriage's nephews might be you and your siblings. You are your uncle's nephew or niece.They might be the children of your aunt's siblings (your aunt being his wife), in which case they are your cousins.If they are your uncle's nephews by virtue of being children of his siblings, they are no relation to you at all.
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
He would be his great great great uncle