Local SEO Google Maps Onebox

Hey Reddit, I have a client operating in a local area here in Germany. To a search phrase like „insurance + town“ he is only in 5th place in the Google Maps Onebox, despite he has with 100 reviews/5 stars the best and most reviews out of all businesses listed. First place has 6 reviews with 4,3 stars. Only thing is, the offices of his competitors are a bit closer to the city center of the town. I know thats a big factor, but can‘t be changed, as they just build a new office. Is there any way to get past his competitors?