The pronoun himself is an intensive pronoun only, it is not reflexive.
Intensive pronouns are used to place emphasis on the subject of a sentence. Here, we want to emphasize that Jason did the whole work on his own. You can easily deduce whether a pronoun is intensive or reflexive if you remove it from the sentence - if the sentence still makes sense, then the pronoun is intensive because they are not essential in sentences. If the sentence no longer makes sense, it means that the pronoun is reflexive.
Here, if you take out the pronoun, the sentence will go like this:
Jason finished the remodeling.
As you can see, the sentence still makes perfect sense, which is why the pronoun is intensive.
