Hi
When you receive a letter saying the application is complete, he'll just have to bring that letter from the visa office to the point of entry (border/airport) and become a permanent resident at that point of entry. There's no necessary waiting period, especially since he won't have to wait for a visa to be issued since he's a US citizen.
Sorry, but Buffalo will send a letter to him requesting his passport. They will return the passport with a Confirmation of Permanent Residence in the passport. He then enters Canada and is "landed" as permanent resident at either the Border or an Airport if he is flying.
PMM