He goes home and gives his dad five hundred quid.
The old man looks at the cash and says, "Thanks, son, this money will mean a lot to me. We’ve never had much in this family, we’ve always been poor. You know, I couldn’t even afford to marry your mother."
"What!" exclaims the son, "you mean I’m.......well.......a b@stard?"
"Yep," replies his dad,
Spoiler: