I'm sorry you have been dealing with this with Kiba and know you will be very concerned for him.
From the details given, it does sound possible that this is linked to separation anxiety and a dislike of being left alone, especially as it is something he was not used to when younger.
Urinating outside of the tray can be a sign of stress and he may have some underlying bladder inflammation (cystitis) triggered by his separation anxiety.
As not yet done, I'd recommend a vet check focusing on this issue that includes a urine analysis. If he does have some urinary crystals, a grumbling infection or a sterile cystitis, we won't get on top of the issue until we address this. Some cats will need e.g. a prescription diet, bladder supplements and/or anti inflammatories.
In the home, things that can help would include:
- Starting some daily calming supplements
- Ensuring he always has access to 2 clean litter trays
- Building confidence with lots of games and mini training sessions
- Cleaning any stains promptly with an enzymatic cleaner to prevent re-soiling
If possible, I would allow him out while you are out. This should hugely help to eliminate the issue. If you can, I'd install a cat flap with a microchip reader.