If your okay with it, long videos of quail talking could probably help too. Mine that were in single cages back cared more about hearing others than seeing them
If you're phasing out of quail, all that will happen with getting more females is having to deal with rehoming or outliving the new ones when he passes. In that case, I think he'll be okay. I had a single coturnix for a couple months after her mate passed away from old age, and she did just fine...