Your great grandson and your husband's great niece are second cousins, once removed.
Your niece's grandson is are your great-grandnephew.
A great niece and a grandson of the same person are second cousins to each other.
Your niece's child and your grandson's child (your great grandchild) are second cousins, once removed, to each other.
Your niece's son is your great-nephew. Your niece's grandson is your great-great nephew.
Your grand niece and your grandson are second cousins to each other
You are his great-great-aunt (or great-great-uncle) and he is your great-great-nephew.* A much less-used US term for the relationship is great-grandnephew which follows the logic that your niece's grandson is your sibling's great-grandson, so would have the same generation-separation from both you and your sibling.
Your niece and one of your grandson's parents are first cousins. That makes your niece a first cousin once removed of your grandson.
Your grandson and your great great niece are second cousins, once removed, to each other
Your niece's child and your grandson's child (your great grandchild) are second cousins, once removed, to each other.
Your grandson and your sister's grandson are second cousins to each other.
great great nephew and u wud be a great great aunt... i think :)
Your niece and your grandson are First cousins, Once Removed.