[matese]

I have to assume the nail grew very long and has turned sideways , the only thing you can do is continue as you are, trim mm by mm weekly but this will take a long time to get the nail to where it should be, be care not to cut the quick. With their black nails it's very hard to see it. You can bring him to your vet, a tech will cut the nail back or try a groomer. It would be a one shot deal, just to quickly shorten that nail then you can continue to maintain it.