Principle of Best Interest applies to child custody

Dear PAO,

I converted to Islam sometime in 2010 and married my husband, who is a Muslim, six months later. Unfortunately, after our marriage, he manifested an unbearable attitude, and he was very jealous. We eventually separated. I reverted to Christianity and now I live with my minor son in Quezon City. My husband is contemplating filing a case before the Sharia Court to obtain custody of our minor son, claiming that I am unworthy as a mother. I would like to know as to what law (the Code of Muslim Personal Laws or the Family Code) should be followed on the issue of custody for our son.