Sorry, if you do the act, you’d better assume getting the girl pregnant is always a possibility, regardless of what she says.
If you don’t want the baby, don’t do the act.
I get that. But (and yes, I know I am excusing him a bit) He was a teenager and stupid, and the girl thought it was the way to get out of an unhappy household, so she lied. Teenage boys aren’t thinking with their brains sometimes.