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He'll be likely to get eye drops/gel, which are usually quite effective. Sorry, my computer didn't upload the picture properly. Good that you have booked him in. Eye injuries/ulcers can come on pretty quickly, but they generally heal off well as long as you get the gel in on a regular basis. If necessary, distract him with his favourite food.
It is possible that he has got a bit of hay stuck under the eye lid and has been scratching at it. If you can find it, you may be able to wash it out with clean cold water. Then; ditto what Wiebke said.