Lord Triggs gives a very good link above. The information is from an experienced breeder who understands the matter and explains it very clearly.
If I understand you correctly ... a fanless environment is not the same as a brooder. A brooder can be made from any small enclosed space, like a fish tank or plastic box, it should be open at the top for air but maybe covered with a light towel to keep the warmth in. Soft cloth, paper towels, old towel or blanket with paper towel on top (so can easily be changed when dirty) on the bottom of the tank or box. A heating pad set on lowest heat underneath the brooder, just on half so the bird can move to where they are comfortable, but you have to check the temperature to be sure it's good.
The formula, you say "cool feed" but formula must be warm, to an exact temperature. We would have it in degrees F on this forum so you have to convert to degrees C maybe. There is a very narrow safe temperature range. If the formula is too cool, it can cause "crop stasis" where food is not digested and in fact digestion stops, and the food in the crop can ferment or become infected. If the food is too warm, it can kill the flesh of the crop and cause an actual hole to open, inside the bird or to the outside. The food must be heated, then stirred well to make sure there are no local hot areas, then check the temperature.
The idea is to feed the baby, and keep it at the proper temperature so it doesn't have to spend any energy trying to warm or cool itself, and all the energy can go to healing. If there are seizures, this could be related to a brain injury or some poison to the nervous system, or some defect of birth. They are very serious. But it's hard to treat them. The best you can do is to keep the baby warm and properly fed, and leave the treatment of seizure to the vet, but it doesn't sound like your vet understands what is causing them, or maybe isn't explaining it right.
When we say "certified avian vet" we are speaking of additional training that a regular vet takes, in addition to the normal degree in veterinary medicine. For this they receive an additional certificate and are listed in the official directory of avian vets. A vet can have a lot of experience with birds but not be officially certified. You can check the list here:
Association of Avian Veterinarians
It includes a field to search by country for international vets, not just ones in the USA.
Do you know if your bird had a fall that might have hurt his neck or back? After you feed him, does the crop get small again in a short time, or does it stay full all the time?
Please don't hesitate to give us details, and pictures or videos if you can, and the experienced people will help as much as they can. I have never raised birds, so I just give you information I've learned from other people here. The people here are very experienced and we care about your baby parrot. I hope things improve for him soon.