Child support?

I didn't know where else to post this but my Fiance has been paying $300 a month in child support for 2 kids from his last relationship. We recently got papers in the mail just stating how much he owes (we get it every month) and we noticed his son is no longer on any paperwork at all only his daughter, so we looked and since May he hasn't been on the paperwork. Why is this? Did she tell the courts he isn't the real father? She did cheat on him when the son was conceived so there was always that possibility. We're just so confused.