It's rude to lie.
Billy has a planned get together with his estranged father, but his friends want to see him too.
Billy wants to cancel, and hang out with friends instead.
Billy tells his father he still wants to get together, but does want to see his friends today.
Billy tells his father he's not feeling up to getting together.
Billy's father understands and lets Bill reschedule.
Billy feels bad that he didn't tell his dad the truth.