I keep imagining OP getting pissed af, IM NOT YOUR DAD!! STOP CALLING ME DAD, SOMEONE MIGHT TAKE YOU SERIOUSLY AND THEN WHAT?!?!?
Chill out pops, really not that big of a deal. There is no ending it as you have allowed it to go on for far too long, you could of ended this easily in the beginning. Kid says that you're his father, you kindly explain to him although you sleep in the same bedroom as his mom, you are not the biological father. But because it has been going on for years, embrace it and make it funny in a less insulting way. Or, and this one is far fetched...maybe try a different court?