Once the nails start to curl I'm afraid the usual non-invasive methods won't work because they just can't contact the surface like they would if they were short. I would highly recommend taking him to a vet. In my experience it isn't the vet who handles the job but the techs and so it's a lot cheaper. I had to do that for an elderly ham I'd rescued. I started cutting and then he realized what I was doing and got squirmy. Not wanting to take his toes off I hauled him to the vet and a couple of techs took care of it. Both of them got bitten though