Welcome. Is the turtle in with any others? If so, it would be good to separate him from them.
Is the entire paw swollen or is the swelling around the nail/s? Is there an obvious wound in the area? Is the skin broken? Are any nails missing? The other feet are normal?
How long has he been refusing to eat?
I don't know where you are, but just from what you've written, to me, the best thing to do would be to find a herp vet and have the paw looked at. There's a post with links to finding one near the top of the Urgent Care forum. To try to find one internationally, if you're not in the U.S., try
www.arav.org click on members and then on the worldwide tab.
In the meantime, if the skin is broken, I'd get some Betadine (povine iodine), dilute it with some water and clean the wound. Keep him in clean water, but do take him to a herp vet.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-