Hello, welcome to YorkieTalk. I'm so sorry that your little Diego is unwell.
I was just wondering.....has your vet ruled out lungworm? It's on the rise in Ireland and the UK and Cornwall is one of the known hotspots. Apparently young puppies can pick up the infection from their mother. Coming into contact with slugs/snails carrying the parasite is how most dogs become infected. Some dogs don't display any outward sign of infection or only present with a few of the many symptoms. Symptoms can include lethargy, coughing, vomiting.
I've included a few links you can read through.
Be Lungworm Aware - UK Lungworm Infection In Dogs Your Dog | Are you lungworm aware? | Dog Health and Care | Dog Care and Advice
Diego probably doesn't have lungworm since he is so young and these symptoms are common in many illnesses but I thought I'd mention it.... just in case.
Best of luck, I hope Diego feels better very soon.