Prayers sent up.
Maybe I missed something?
Is he still nursing from the mare? Did you milk her out to get rid of the colostrum after you found out that he is jaundiced and had neonatal isoerthrolysis? Once the colostrum is gone it is my understanding that then you can let the foal go back to nursing the mare. It is the colostrum that contains the antibodies that cause destruction(in this case) of the foal's red blood cells which then makes the foal anemic and can kill the foal.
Robin