A.P.
CPS folks are actually pretty good folks.
My suggestion is to call the supervisor of the person assigned to your case (the case worker.)
If you have all of the medical information available, send them a certified copy. A letter from your son's own personal pediatrician would also help as an introduction, with a description of his disorder and signed by his own hand.
If you do not have a full record of your son's medical history, get it asap so that you have it on hand at all times, in multiple copies if necessary.
Also, if you change doctors, that's a really good reason to access it.
Plus, have your child re-evaluated for failure to thrive and the other medical issues, so that you have fresh evidence.
Once you have been targeted by CPS, it's pretty much a lifelong target, and they examine everything to make sure they are not lax in their duties. But don't get discouraged, they really do want to do their jobs right.
Trust me, they see babies all the time with failure to thrive and other issues that cannot be explained by the mothers, so they get suspicious unless there is a parent who is on the ball. They have to trust that the doctor or the other person who was concerned enough to contact them was educated enough to call them and inform them, and they have to follow up in case something really is wrong. Lots of kids slip through their fingers because of huge case management loads, so you are actually (sadly) one of the lucky ones getting their unwanted attention!
Just to put your mind at rest a bit, their goal is not to split the family or take your child away. I promise. Some of them might be old school and kind of dumb, but for the most part they are very highly trained and very sensitive to their communities. Just watch for a burnt out worker, that's when you really have to call the supervisor. It might even help to visit the office and talk to some people there, just don't bring your son with you and dress up a bit. If you still do not get anywhere, you can file a grievance with the county to start the ball rolling on a hearing.
Good luck!