Thank you, bookbug.
I did send a follow-up letter that was friendly in tone but asked some questions, such as "How come you never said anything about my engagement?". Still, no response. I think it mostly just came at a really awkward time, since he'd been planning to propose for at least a month prior to our re-establishment of contact.
I may actually call him in about a month to give them time to have their newly-engaged joy in the hopes that we can actually have a conversation. By that time, I'll be ready to send out my Save the Dates anyway and I can just ask him whether he would feel uncomfortable going.