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What sort of illness did he potentially have Katsunae? If he doesn't look right to you it is likely that there is something amiss. Is he easing and drinking OK? The dehyration is worrying as this is definately not normal. I'd suggest you use a small bowl in case he is unable to use a water bottle. I wonder if he has a cleft palate? Can you see a gap in the top lift or the skin divided in that area? It is quite a serious condition if he has as often they are unable to feed properly and food passes in to the cleft and causes other issues.
He could have received an injury while young and more commonly have maloccluded teeth which could be growing wrongly and piercing the gums. Can you check the teeth in any way? Does the pet store have a vet they use who may be able to check him over? I'd try doing the health checks and ensuring he can take water from some sorce. Maybe small chopped cucumber or oats mixed with water?
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