Hi Sam,
You made a common mistake, but you should be able to regain his trust with some time. Birds are prey animals, so they do not like being grabbed. In order to trim nails, you have to hold the bird in a way that would mean death if you were a predator. The owner should never participate in grooming. Even standing close and trying to console a bird can cause issues because the bird is scared and panicking, and is seeing your face, so he associates it with the grooming. I recommend taking a bird to a professional to be groomed. They know what to do if the bird bleeds or has a stress related issue. Do you even have anything on hand in case a nail bleeds? I always tell the owner to stand away and not speak. Once the bird is groomed, and back in his carrier, the owner can approach the bird and reassure him. By taking the bird to a professional, he doesn’t associate the grooming with anything in his environment.
What you need to do is spend time with him just talking to him. Don’t reach for him or even show him your hands. He may settle back down in a few more days. But if he continues to be skittish, go back to just talking to him. You need to wait until he feels safe approaching you. When you get to the point that he seems calm enough to pet, don’t reach behind him. Touch the front of his neck or the side. Predators attack from behind, so it may take some time before he trusts you to pet him again. Just take things slow and keep being patient with him.
Thank you for asking Lafeber,
Brenda