How can my son petition his dad in the Philippines if he is a US citizen?

Generally a U.S. citizen age 21 or older may sponsor a parent located abroad through "consular processing" in the immediate relative visa petition. There are many details that are important for determining eligibility, however. It would be wise for your family to consult with an immigration attorney who, after learning all of the relevant information, could advise about eligibilities, options and strategies.