Well, the vet rang at 8.30am and said his electrolytes are unusual and they are pointing towards Addison's Disease
She said it was unusual in such a young dog and we are going in for another test specifically for it at 10am. TBH, as he's still shaking and not eating, I'd rather had a solid diagnosis so he can start his treatment (for life) which shouldn't reduce his life expectancy. Will update you all if he does have it later as he has to stay at vets for a while
Thank you again for your kind wishes and thoughts, they really help