Currently I don't believe there's something END related that's active in Natsu. Igneel was very specific in his instructions to Natsu about not opening the book of END so that indicates the trigger is within the book. For now, for the sake of keeping things simple, I'm going with the fact that there's no need to worry about END emerging as long as the book isn't involved. Also, Natsu wouldn't be able to have access to Igneel's power if it wasn't specifically left there for him. Igneel and the other dragons were literally within their kids for years but their power never manifested nor were the DS able to specifically use the dragon's power. They could only use that which was taught to them and expand on it. I'm pretty sure Natsu wouldn't be able to use Igneel's power in any context unless the latter permitted and made it accessible to him. For me, this also cancels the theory of Natsu using the antibodies left behind or the power suppressing END. I don't even think Igneel's power is suppressing END. Igneel's power is destructive in nature and not a seal.