Can i ask child support if parents made an agreement?

Hi! I am just confuse what to do because I and my ex boyfriend have 2 kids. Our son is 7 yrs old now and our daughter is 3 yrs ol. Our son carries his father's last name and the 2nd one carries my name. Before my son turned 7 I get the full custody of my 2 children. He made an agreement through DSWD that he wants to waive his responsibility, financial and morale obligations. He also stated no communications and no visitations. I have no choice but to sign just to make everything easier for me to get my kids from him, 'coz i used to work away so I left them under his supervision. But I never cease to communicate and support my kids. Now that my son is 7 years old he showed up sometimes he visit my son at school without my awareness. Can I file a child support even though I signed the agreement he made? But I dont want him to borrow the kids because as of now he already have a live-in boyfriend since he is a bi-sexual. What should be my first step? Thanks for the advice.

yes, you can ask for support. waivers of future support are invalid.

thanks! Now I know where to start.