He doesn't actually cum?

I've had sex with my boyfriend once and he seemed like he was coming and we kept going until it seemed like he had actually cum, but he hadn't. He wasn't even sure until I got off of him that he hadn't actually cum and I'm just confused because I didn't know it could fake out like that. It always seems like we're on the cusp of getting it right but it doesn't actually happen.

(PS. I know he can because we've done oral, I'm just confused and I don't know if I'm doing something wrong.)