He is getting 1% calcium in his chicken feed, I assume. That is what nonlaying birds need. It is good that the vet is hopeful that it is not Mareks, but I am afraid that it may well be Mareks. It is everywhere in the world, and the neural form is usually the first type seen.But he should have it at disposal, no? Since he can't forage anymore... the chicks don't take it, they find enough through the garden
Chicks from hatcheries are not vaccinated unless the customers ask and pay extra for it. Some people do not want to or know to vaccinate. I have both vaccinated and unvaccinated birds together, and none have ever been diagnosed with it.