It took us almost two years after we started my son's strict diet before the tics finally went away completely. Hang in there. Don't stop the diet. If he is reacting to corn that strongly then you know you are on to something. You have to remember that antibody levels rise and fall SLOWLY. So it could be that your son has very high anti corn antibodies right now and so his tolerance is non-existent. It could take 6 months to two years to get those antibody levels down to the normal range. We have seen this with our son's gluten antibodies and the stool testing we do annually. After two years he still had elevated antibodies, although they were really close to the normal range.
Don't quit, don't quit, don't quit.
Now my son can have minimal corn without any tics. The diet takes time. That is all I can say. We monitor and limit the corn to 'special' occasions and he seems fine.
We do NOT do any corn byproducts, syrups, or any artificial colors at all.
I want to add that corn syrups, in particular HFCS, has mercury in it. Might be a factor with all these kids having neuro flare ups when they eat it regardless of corn allergy.