Keep in mind that the IRS looks at things "literally" when it comes to taxes. So if you are not the real father, he can't be your son unless you went through the formal adoption process. That's how the IRS sees it. Now here's some things that matter, and I ask this stuff because there are so many possible scenarios, I'd have to write a book to cover them all. So lets see what specifically applies to your situation.
Where is the mother and father of the child? Generally they get first dibs on their children when it comes to taxes. As a non-blood related relative to the child, you are last in line. So this really matters.
You say he "lived with me and *his mother*" Are you married to his mother? Is she just your girl friend? Is that his biological mother? If the latter, why is she not claiming the child on her tax return? Maybe she didn't work at all in 2017? Maybe the biological father has the right to claim him under a divorce decree? This "can" get deep, and if you claim him and your claim is not legal because of other legal agreement that does not involve you, then you can potentially find yourself in some pretty deep kim-chee. So all this needs to be known.
