I'd like to know whether that sentence is grammatically correct.
To my way of thinking, that doesn't sound quite correct. If I were the writer who wrote that sentence, I would say as "Collecting stamps is one of his hobbies"
But If we could think of "a hobby" as modifying the whole sentence "he collects stamps", I'd think that that sentence could be grammatically acceptable.
I'd like to know your opinions on my thinking.