Your puppy is probably big enough already to swallow those necks whole without much of an issue. You could try holding on to them and force him to chew a bit. They are probably small enough that it wouldn't be a choking hazard either. But I wouldn't start out giving the necks because if he does swallow them whole, his body is not and won't be able to handle that much whole bone yet. Remember that his digestive secretions have to change to the point where they are capable of taking care of bones like that. Should take a week or so for his body to function correctly to digest bones. This is the reason why almost all newly switched dogs have pieces of bone in their poop, but experienced raw fed dogs can handle whole rib bones and be fine.
Keep the necks around until he has been on raw for a few weeks and then give them. I would look into getting chicken quarters if you can and alternate those with the chicken backs since quarters are more appropriate for his size.
