Can I petition for a fiancé visa for my partner of 5 years and her 8 year old son in the Philippines?

Significantly more information is needed in order to provide a legal analysis of ways you may be able to sponsor your domestic partner/fiance and her son for immigration benefits. Note that if she now is located abroad, a marriage ceremony in the U.S. without her may be invalid. A fiance visa is only available to those engaged to be married to U.S. citizens. Only some types of nonimmigrant visas (like an H1B visa) would permit your partner/fiance to enter the U.S. with the intention of marrying you and then staying permanently, and misuse of other nonimmigrant visas (such as a visitor's visa) could have very harsh immigration-related consequences. There really is no substitute for you and your fianc?e to consult with an immigration attorney who, after learning all of the relevant information, could advise about eligibilities, options and strategies to attain your goals, and who then could offer legal representation in the often complex application process.