Cat Eye Problems

[caitmac]

My 8 year old cat is generally very healthy but has not been opening his eye fully for the past week at all times. He lets me have a look at it so I don't think there is much pain and there is no puss or crusting, but it is still worrying me. Anyone have any idea what it is? Would like to know if it will pass or whether i need to bring him to the vet
Thanks