。
Building model airplanes is his only pleasure.
Literal
Model airplane [object-を] make [nominalizer-こと] [subject-が] his [possessive-の] sole [possessive-の] pleasure is.
こと-nominalization rather than の-nominalization this time, attaching to a more abstract or fact-like activity. 唯一 ('sole, only') is a Sino-Japanese compound commonly used attributively with の. 楽しみ ('pleasure, enjoyment, something to look forward to') is the noun form of 楽しむ ('to enjoy').