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Can you pick the bird up and examine it for injuries? Look at the legs/feet/toes.
A better photo of the legs and feet may be helpful.
What have you been feeding him/her?
I agree that it looks like a meat bird. Sadly these birds are not meant to be long lived and gain a lot of weight in a short period of time unless feed is managed/controlled. They can end up with leg issues where they simply are not able to support their weight, also with the rapid weight gain they can also have heart failure.
I would try to rule out injury for sure, but it may be that your bird is failing due to weight gain.
What does the poop look like?
I would get some vitamin B2 into it as well. You can give B-Complex (1/4 tablet) for a week.
The corn, I would take away. That is just going to add to the weight gain. IF you can get him/her up and moving, then you can try using a grower/finisher type feed which is lower protein. Only put out small amounts and encourage the bird to move/exercise. This may or may not help. I'm very sorry. Even those that have these birds to raise as pets and closely monitor their intake and encourage exercise find that they are not long lived. Some do live for a year or more though. I don't mean to sound discouraging. My understanding is these birds can be very sweet and are like big pets, but I do have to be honest with you too.