You can't file jointly with your girlfriend, since you're not legally married. You may be able to claim her as a dependent, if her income was under $4050 in 2016. You should be able to claim your son as a dependent and file as head of household. However, you'll also have to meet the other requirements below.
https://turbotax.intuit.com/tax-tools/tax-tips/General-Tax-Tips/Guide-to-Filing-Taxes-as-Head-of-Hou...
