Closing one eye

My cat started today closing his left eye I am already worried.

What a stunning cat - and you are right to be worried about this one.  It looks to us like a slight eye-crease, but you have perhaps astutely realised that cats tend to avoid showing pain until it is unavoidable; showing pain does not always win a cat social sympathy with other cats in the way it might do with a dog in a pack.  The upshot of this is that evolution has given them more discrete signs of pain, which humans tend to miss - but screwing an eye is usually a strong sign of ocular pain and eyes, when threatened, can go downhill very quickly.  Reasons for eye pain can include dryness, ulcers or other damage to the surface, pressure change, blockages, internal tooth danage, lumps and bumps in the uveal tract and more.  Your vet - and perhaps an opthalmologist if they are lucky enough to have one - might want to examine this more closely, sooner rather than later.  Please contact them as they may treat a unilateral (one sided) ocular discharge as an emergency.

There are some occasions in which a 'squinty' eye is not painful - and these are ones where the nerve supplying that area has been damaged.  However, this is rare and when it happens can be acute (starts suddenly eg in connection with a partial seizure or a fit) or chronic (often progressive and) longer lasting.   However, either of these would also need medical attention.