It seems my son’s father is making him attend college just to get payments

My son is 17 yrs old and lives with his father, our daughter who is 20 (a university student) lives with me on weekends and holiday time. I pay CSA for my son who is in college part-time, is this fair seeing as I keep my daughter even though she is older? Also I have now been told that my son does not attend college regularly, and it seems his father is making him attend just so he can receive my payments.