K-custody issues are so hard and full of game playing.
You need a good lawyer, you need documentation of the illnesses,it will be hard because unless you can prove abuse it usually goes to the mother no matter what it says under the law
If you get a sympathetic judge they might listen under the argument that yours is the more stable envirnment but I have seen it over and over- this will be a problem for you for the next 14 years
Love that little boy like you do and don't give up