Just as a bit of added help since you can’t ask your father which way round it is.
Either:
Your father (Dave) has a full cousin (Jack). Dave and Jack are first cousins.
Dave has a child (you). Jack has a child (Marshall). You and Marshall are second cousins. Dave and Marshall are first cousins once removed.
Marshall has a child (Simon). You and Simon are second cousins once removed. Dave and Simon are first cousins twice removed. Simon is your father’ full cousin’s grandson, and therefore likely to be considerably younger than him.
OR
Your grandfather (Wilfred) has a full cousin (Sid). Wilfred and Sid are full cousins (first cousins).
Wilfred has a son, Dave (your dad), and Sid has a son, Jack. Dave and Jack are second cousins and likely to be around the same age (not guaranteed though).
Dave has you. You and Jack are second cousins once removed.
Hope this helps but can answer any questions if it’s not clear.