If the contacts were to be still in the eye after four months of not removing them, the eyes would not have been normal at all, even sleeping for few hours contacts with the contact lenses
on can lead to severe corneal problems and the cornea gets swollen and presents with reduced vision
, pain , photophobia. Since his eye has become red since one day only , it may not be due to retained lenses
( which you have already verified ). But a detailed eye check is necessary to rule out a corneal involvement in known history of contact lens wear. Till then he can instill lubricating eye drops like Refresh tears , 1 drop 4 times a day for soothening effect.
It is better to get his eyes examined by an ophthalmologist
for the needful ( slit lamp examination ).
Thanks and best wishes
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