It's really hard to offer thoughts on where you stand without knowing more. Not asking you to divulge...just making point that none of us can know the likelihood. What I would say is this. You sound quite vulnerable yourself and need some support in your parenting role (which you've said) and certainly support legally to protect your rights. Please don't sign or agree to ANYTHING without proper legal and independent advice. Social work should be about keeping families together, unless risk to child is too high in which case they would have to go to court to remove a child, so if they get involved just ask themfor some help to keep him, show them how you can do this...but maybe a little respite every so often, or support worker could help?.
They will be looking at risks, being afraid of vomiting is no reason to remove a child in my eyes, nor is a my diagnosis. It's how you are around him that matters. How do you cope when you are not feeling well, or when child is sick.
I think anyone who gets angry like your ex's partner is not best placed to take your child. Tell social worker what's she's said/done. You've had your son 5 years already, so you can do this. What helped you get through the tough times before? You have strengths ...use them.
We all feel useless as parents sometimes, that doesnt mean you are. There's no such thing as a perfect parent.
Good luck, don't let people make you feel bad, your son loves you and you him, there's a way to make this work x (I just wouldn't be involving ex partners ex in any formal arrangements again myself!)