Surely Achilles's concubine would come around. His love for her was legendary, after all. Neoptolemus felt secure in that, and didn't question it further.
He was happy to go wherever his father directed. Just as Achilles didn't quite know how to be a father, Neoptolemus didn't have much to judge by how parenting went, so at least of Achilles messed up he wouldn't be able to call him on it.
Neoptolemus felt comfortable in the companionable silence. He wasn't prone to filling the silence, himself, so he didn't think to talk about a different subject. He was just... happy.