Your contractor did you no favors. What he did is plain wrong. A ceiling fan box must be mounted securely to solid structure, not hang it from the drywall. In a retrofit application like yours, the easiest thing to do is buy a kit as shown in the attachment below. It will consist of a threaded bar assembly that passes up through the standard hole cutout in the ceiling and is then unscrewed between joists above the drywall to clamp it solidly in place. The ceiling box then attaches to the bar assembly. Simple. Easy. And the correct way to hang a retrofit fan box.

https://www.homedepot.com/p/Commercial-Electric-4-in-Round-1-1-2-in-Deep-15-3-cu-in-Metallic-Ceiling-Fan-Light-Fixture-Box-and-Brace-Kit-for-Old-New-Work-CMB150-OB/205383178

This old-work (i.e., clips to drywall) ceiling electrical box has a capacity of only 6 lbs. You can use a fan box as described above. If your joist is in the right place you could also use this clever 4 in. Ceiling Fan/Light Fixture Side Mount Box for Old/New Work.