Your cousin's son is your first cousin once removed. Since you and your cousin are in the same generation and your cousin's child is in the generation after yours, he is removed once from your generation. This means that your grandparents are the child's great-grandparents.
Your cousin's grandson is your first cousin twice removed. Your parents' first cousins are also your first cousins once removed, since your parents' generation separates you. Your parents' second cousins are your second cousins once removed, and so on. If you have children and your sibling has children, these children are first cousins to one another.