Child Custody (for unmarried couple)

Me and my boyfriend lived in together for more than a year in cebu. We had a son who was born december 15 last year. I needed to leave for manila on my 8th month of pregnancy since there will be no one to care of me in cebu because my family is living in manila, and my boyfriend works everyday except weekends. We just broke up last january first of this year when i knew he's cheating on me. He had a girlfriend right after i left cebu, but they broke up when the girl knew about me and our baby. He's presently having another one right now.
My x-boyfriend is not supporting the baby. Good for me that i have a brother who is still single and professionally working. He's the one supporting for all the needs of my baby. Of course, i should not be contented with this set up. I have to work. But i cannot work yet since i'm taking care of my baby. My mom cannot take care of my baby anymore because she's having osteoarthritis and osteoporosis. She's now old too. I already thought about having a baby seater, but i have no money yet and stable income to pay for that.
Now i decided to give my son for the mean time to my xboyfriend's mom...and planning to get him back when i get to save money and stable income after a few months. I tried to contact my x-boyfriend and told him about my plan, but then he told me that the moment i give my son to them, i wont be able to get him back. I tried to text his mom and told her my plan and she agreed. She agreed that i can get my son back as soon as i'm already capable of taking care of him already. What i'm worrying about right now is that, what if i wont really be able to get my son back? What if, when i'm already prepared enough financially to take care of him, they will keep him somewhere and just tell me he's missing and they do not know anymore where he is?
My friends told me to make a notarized letter of agreement have my x-bf's mom sign it. And of course i have to sign it too. So that if ever they will hold my son back, i can appeal to the court for my son's custody.
My questions are, what should be that letter contain? If ever i get to make such letter, would that be valid? Will the court recognize my letter if ever i will appeal to them?

Please help me with this. I just really need some help....badly need help as soon as possible. Thank you so much in advance!

There is no need of executing a letter or an agreement to that effect, since the law already grants the custody of an illegitimate child SOLELY to the mother. In case your x-bf's mom will unlawfully witheld your child or renegade in your agreement, you can file a petition for habeas corpus in court. Just present the birth certificate of the child and your NSO regarding your "single" status.