B12 is only vitamin B12whole B12 complex tablet
B Complex has all the B vitamins (there are several)
Make sure it is B Complex.
A bit of a Vitamin E supplement will help with absorption of the B Complex.
A little egg will also add some Selenium which I was told was useful with this issue. Selenium supplements can build up in the system so the egg is better for him to get this.
B vitamins are water soluble so way harder to overdose these.
He needs the B Complex and E vitamins in a therapeutic dosage which is more than what is in normal "chicken vitamins" that we mix with water. When my hen was having this issue I gave the supplements and put chicken vitamins in her water. I was fortunate and she recovered. It took a few weeks, maybe 3???
Hope he improves. Prayers for King.