Does the biological father have to pay child support if signed his obligation away while he entered the Army?

My boyfriend of 4 years had a baby with his ex girlfriend about 5 years ago. He made her PROMISE that if she got pregnant, she would get an abortion. Well, she got pregnant and went ahead and had the baby. After he was born, my boyfriend enlisted in the Army and signed a paper from the court stating that, "He is not the legal guardian and gave up his rights as a parent" of his baby (his ex signed it too). Last summer while he was unemployed the court sent a letter requesting child support from him. He put ZERO income for everything and he never heard from the Court again. We are hearing from the his ex girlfriends stepmother (who is taking care of the baby and who took legal guardianship of the baby) that she wants to seek child support to come after him now since he is currently employed. His exgirlfriend currently pays about $300 or so to child support but he has not and does NOT want to. The promise that she made to him meant everything to him and when the promise was broken, he felt tricked/betrayed into fatherhood. Please help me and give me advice on whether or not he still has to pay child support if the Court comes after him! Thank you.