I'm sorry. I think Little Brown Dog gave wonderful advice, especially the part about making sure this is considered a consensual relationship (with the age difference and your son's developmental issues). If it isn't, that will give you a lot more leeway regarding custody of the child and keeping your son away from her (if she's considered a statutory rapist, a restraining order should be fairly easy to get, for example), and can also be leverage to get her in line (whatever that means to you and your family). Also, though this news is huge and heavy and I'm sure you feel the need to figure it all out right now, you do have time, so try not to push yourself to make all the decisions Right Now. You *will* get through this. And you do have time to consider options, especially for things like long term involvement with the child and with your son. So try not to get overwhelmed with all of this. Right now, the alligator closest to the boat seems to be your son's living arrangements, and the rest can probably wait a bit.
I believe you said they started dating in December. She now knows she pregnant, so she has to be at lest ~1 month along as of mid-February, if not more. That's a fairly tight timeline, and does make me question whether she might have already been pregnant and gotten involved with someone so she could pin it on them. I wouldn't be terribly surprised if this is an 8 month "premature" birth. That may well not be true, but the timeline is concerning. If she's not willing to do an amino or the blood test, you could at least ask to attend a doctor's appointment and/or get a copy of the sonogram photo, both of which may give you a better (though still not exact) estimated conception date. You only have leverage if there is something you can offer that she or your son want, but even just getting your hands on a sonogram might be enough to reveal possible timeline issues, and could probably be done without telling them exactly why you want it.
I have little else to add to what LBDBut if it helps, some stranger on the internet is aching for you and thinking about you and your family.
In addition to legal counsel and counseling for your son if he's willing, you might also consider seeing someone yourself, just so you have an outsider to help you sort through and process all this, and all the many choices you are going to face in the coming months.