SanDisk Clip Sport Plus Bluetooth problems

My son has had the SanDisk Clip Sport Plus for about a year and had no problem with his bluetooth - it always paired with his headphones (Sony).

A few weeks ago he tried to pair it with our bluetooth bose speaker and since then he’s be unable to reconnect with his headphones. We’ve tried:

Factory resetting the MP3 player

Formatting the MP3 player

Updating the firmware on the MP3 player

Resetting the headphones

Currently the MP3 player appears to find the headphones, it says that they are paired, the headphones indicate that they are paired - however no sound comes through when he plays anything.

The headphones work with mobile phones (so they are working). The Sport Plus does the same now with our Bose speaker i.e. seems to be paired (the speaker even says ‘connected to Clip Sport Plus’) but no sound comes out. I have tried uping the volume!

Realise there have been a few older posts on this subject but wondered whether there were any recent bits of advice.