Where was Nicholson born and raised?

From what I can see, you are referring to Brian Nicholson. To answer your question, Nicholson was born and raised in Kenya. During his youth, he attended the Prince of Wales School in Nairobi, the capital of Kenya. The author, Peter Matthiessen, describes Nicholson as aloof and brusque in manner.
In the book, we discover that Nicholson was a former warden of the Selous Game Reserve in southeastern Tanzania. When Nicholson flew Tom Arnold to the reserve in 1976, he was grieved to discover that the system of dry-season tracks, air strips, and patrol posts he had established between 1950 and 1973 had fallen into disrepair.
Despite Nicholson's curt manners, Matthiessen found that the former was extremely knowledgeable about the ecology of the Selous Game Reserve. According to Nicholson, the Selous Reserve can lay claim to having East Africa's largest populations of lion, hippo, buffalo, leopard, rhino, and crocodile. Nicholson also maintained that the Selous Reserve is home to more than three hundred and fifty species of birds and two thousand species of vascular plants.