I think Dart was feeling a little lonely when he wrote this letter. There was not much to say as he awaited the time to call home in hopes of talking to Dot and his family at the same time.
He’s sorry she’s going so far away without his getting to see her first. Bear in mind that she plans to do her semester of work experience in the east and only come back to Cleveland in February for graduation weekend. Beyond that, she has no plans to be anywhere near where Dart or his family are. I’m sure Dart has concerns that they may drift apart, given nothing concrete to hold them together, but he doesn’t mention it.
At last he stops to make the call and returns to the letter after it’s over. Although thrilled, as always, to hear her voice, he was shocked to learn that she’ll be heading home in the next day or so. He ponders that she’ll be having great weekends at home with swimming and family activities. He asks her to tell him all about her trip and about her hometown. He says, “Write a travel folder on Greenwich, sell it to me. (As if I’d need selling on any place you might be, and especially where you live.)”
He tells her that wherever she goes and whatever she does, his thoughts are always with her.