What I've heard is that 10% of a bird's weight is blood, and they can safely have up to 10% of their blood drawn for testing. They can survive greater blood loss, though. So... a 400 gram Amazon could safely lose 4 grams of blood or a little less than a teaspoon. If his foot has not stopped bleeding, then you really need to get him to the vet as soon as possible... even a non avian vet could probably patch up his foot in an emergency. Until then, try to stay calm, it will help him to stay calm and minimize blood loss.. put him in a smaller cage or carrier and cover the sides to help keep him calm. Make sure he has plenty of water, offer some juice or fruit if he likes that also.
If the foot has stopped bleeding, then just keep him calm and check in on him every now and then to be sure it hasn't started bleeding. Keep an eye on his foot. A vet check is still a good idea to be sure it is healing well.