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How long has he been having them?
Does he suffer from migraines?
These symptoms could be due to ocular migraine(may or may not be followed by headache), retinal artery occlusion, posterior vitreous detachment etc. A dilated eye examination is important to say anything with surity. Sonce, we do not know what it is it would be safer to get this checked out early as some causes are serious. You should see an ophthalmologist for this. Thanks and best wishes.
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