I just did a quick search on it and from what little bit I read it seems to depend in what caused the Horners syndrome. It could also not be related at all. None the less he should be evaluated immediately.
Below is an excerpt from this: http://www.eyevet.ca/horner.html
Horner's syndrome is associated with damage to the sympathetic innervation to the eye. The damage may have numerous causes, and may occur anywhere along the course of the nerve's route from the brain to the eye. Thus Horner's syndrome may be associated with (strictly in anatomical sequence - not probability) brain tumours, spinal cord injury in the neck, thoracic tumours such as lymphosarcoma, injuries to the neck from fighting, choke collar injury or difficult venipuncture, middle ear infections, and viral, immune mediated or idiopathic neuropathies.
Of all dogs diagnosed with Horner's syndrome 90% or more will be middle aged to older Golden Retrievers. Cocker Spaniels are the second most commonly affected breed.
In most cases of idiopathic Horner's syndrome spontaneous recovery will occur in an average of 16 weeks. A thorough physical examination is warranted to rule out any of the other causes of Horner's syndrome. If the veterinarian has any suspicion that the cause may not be idiopathic, then it may be prudent to do blood work and a chest x-ray, and there is a pharmacologic test done by the ophthalmologist which may further localize the site of the lesion.
If the damage to the nerve is in the sympathetic ganglion (just behind the eye) or beyond we refer to it as post-ganglionic. Most cases of "idiopathic" Horner's syndrome are post-ganglionic, and these cases have a more favourable prognosis. If the damage to the sympathetic nerve is anywhere between the brainstem and the sympathetic ganglion we refer to it as pre-ganglionic Horner's which is prognostically more serious. Most veterinary ophthalmologists can tell the difference between pre and post ganglionic Horner's syndrome by doing a pharmacologic test which tests the response of the pupil to the application of different medications. Sent from my iPhone using Petguide.com Free App