Translation:The boy has sat himself down on the road.
Is the "sig" obligatory here? If so, then the reflexive should be optional in English because we can say (and normally would) "sat down" rather than "sat himself down."
Sat is the past participle of the transitive at sætte, so it needs an object. He "has set himself".
You could also use the intransitive at sidde whose participle is siddet, with the slightly different meaning that he's been sitting there for a while: Drengen har siddet på vejen. - The boy has sat on the road.
It requires an object in Danish, but not in its English translation, however.
What does this mean? That the boy plopped down in the middle of the road?