Reverse tethering via Bluetooth: How to share PC internet connectivity with android phone via Bluetooth
This is a good question which myself  couldn't get a satisfactory solution. This solution posted here seemed promising although I found out the bluetooth stack supports fewer bluetooth adapters and also some link was taken down.

Having said that, since you are running Windows, Microsoft seems to have at least provided a solution to that issue, but it supports newer editions of Windows (should be Creators update IIRC):


  
  Open the Settings app either by clicking its icon on the left side of the Start menu or simultaneously pressing the Windows logo + R
  keys.
  In the Settings app, go to Network &amp; Internet  > Mobile Hotspot page.
  To turn on mobile hotspot, turn on the option labelled Share my Internet connection with other devices. 
  Under share my connection over, choose bluetooth option.
  


Simply pair the two and enjoy the connection



Credits (and photo) Mobile hotspot or Wi-Fi hotspot in Windows 10

Windows feature are most buggy for me, that's why I prefer to try other workarounds first.