A few years later, I did this on Raspberry Pi 4B, maybe this can help someone.
Steps: 1.Make Pi's Bluetooth discoverable and pairable. Create a bluetooth-pair.sh file with content as below at /home/pi/AutoStart
sudo bluetoothctl <<EOF
discoverable on
pairable on
exit
2.Correct the Bluetooth spec file to make time limit to nolimit.
sudo nano /etc/bluetooth/main.conf
set timelimit to 0
DiscoverableTimeout = 0
PairableTimeout = 0
3.Let the bluetooth-pair.sh file to be run automatically when Pi starts. Correct /etc/rc.local Add
sudo sh /home/pi/AutoStart/bluetooth-pair.sh
before
exit 0
This works on Pi 4B normally.
https://raspberrypi.stackexchange.com/questions/47200/automatically-accepting-bluetooth-connections-on-a-pi-3