What is the next step and how long does an I-130 petition take?

Usually, the applicant will need to prove that they lived with the adopted child for at least two years. This evidence needs to meet the burden of proof. The burden of proof is the level of evidence needed to satisfy the government in its discretion. Yet, because you're from the Philippines and you are a lawful permanent resident, there are very limited visas. This means that your son will likely fall in the F2a category if the petition is approved. You should seriously consider becoming naturalized US citizen this as soon as possible. If you have any further questions, that I strongly recommend an appointment with a competent and experienced immigration attorney before there are any complications.

The USCIS is inconsistent in the amount of time it takes to adjudicate a Form I-130 application following its issuance of a Request for Additional Evidence and a petitioner's filing of a response to that Request. Sometimes the USCIS will complete its adjudication within a few weeks, and other times it takes multiple months. While it would have been best to have engaged an immigration attorney at the onset of the case to assure that the application was properly prepared and fully documented, since that is likely to have eliminated the need for a Request for Evidence in the first place, at this point it would be best to work with an immigration attorney to assure a proper and complete response to the Request. On a related note, be aware that the May, 2015 Visa Bulletin issued by the USCIS shows that visas are currently available in the Family-Based Second Preference (FB2A) visa category - the category by which a Permanent Resident may petition for a child under age 21 - for cases with a Priority Date on or before the beginning of September, 2013 (there is a slightly longer backlog for Mexican nationals). When visas become available for your child, you will be able to go to the next step: consular processing. Some immigration law firms, including mine, offer legal services on a "flat fee" basis so that a client will know the total expense from the very beginning, and a few immigration law firms, including mine, offer an initial consultation free of charge.