It's a complement to the verb. I don't think it's of the kind that's called a 'subject complement', which is one that can be equated with the subject. 'He is the park keeper' means he = the park keeper. (There, 'the park keeper' is a complement of the verb but also a 'subject complement'.) Instead, 'in the park' is just a property of him; it's not what he is.