NO COOKED BONES EVER!!!!
:D
Turkey necks are fine raw, but if you cut it into pieces be sure that they are too large for him to swallow whole; quite a few dogs have choked to death on turkey necks.
Are you feeding a raw diet already, or kibble? I would not add the turkey neck to the kibble, feed it as a separate meal (or two!) The giblets would also be good; those you might want to cook lightly just because they can be very rich and may cause digestive upset.