How old is he? I don't think his neck is the cause of the balance problems but rather the effect of them. He has his neck lowered to help balance him. I don't think his neck is too long it just appears that way because he is extending it.
Sometimes it takes awhile for newly hatched chicks to figure out what to do with their bodies., like a new foal getting used to standing on long legs. Is this the only chick you have exhibiting this behavior? How does he act otherwise in regard to eating, drinking, etc.