He absolutely fucking understands needing time alone, so he just nods.

He can't say out loud what Becca was, can't make himself say the words, but he can fumble out the little notebook she left him-- about four inches tall, just barely small enough to fit into a pocket, with a spiral binding and a leather cover with a ribbon to keep it closed-- and turn it to the first letter, one of the two places he has dog-eared. It explains that "this is Becca, your sister" and her last understanding of him being dead and her being so sorry for everything that happened. And then that she's going to leave him lots of notes while she's here, which is presumably what fills up the rest of the little book.

Ellie can read however much she wants to, of course, but that letter is the bit B's using to answer her question.