So sorry to hear about Jethro
Like others have said he is in the best place.
Willow had steroid responsive meningitis which too is an Immune mediated disorder where the immune system attacks itself. Its lucky they have been able to recognise what is it straight away. We were not so lucky with willow as our regular vet was away. By the time our vet was back and said shat she thought it was willow couldn't move and was in a very bad way.
She was treated with steroids to suppress her immune system and made a full recovery. We were referred to 'the willows' referral centre in solihull where they specialise in internal medicine. All the tests and treatment were carried out there by a specialist that specialises in immune mediated disorders. They acted quickly.
Paws crossed for a full recovery i feel your pain