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Hi, this is Dr Hill. Sorry he is having difficulties. It sounds like the dry food is an irritant. Has he been seen by a veterinarian and had his teeth checked. Cats get resorptive lesions in their teeth where holes develop and expose painful roots. He may be able to eat canned food but when he ate the dry it hit an area that was very painful. If he is not eating food, then he needs to be seen by your local veterinarian. Since he was a stray, he will need a good physical , felv/fiv test and deworming as well. Feel free to ask additional questions. If you are satisfied with my answer, then please take time to rate my response. Take care.
You are very welcome.