Thanks to you too for your reply diggingdeeper.
My Mum and Dad have just got back. They took him to Birkenhead Vets in Livingstone Street. They examined him, took his temperature and some blood samples.
But his condition worsened to the point that he was completely motionless and unresponsive, his eyes were glassy and just staring unfocused. Given that earlier he had seemed to be in pain, crying and vomiting, (or trying to,) my parents made the decision to end his suffering.
We are all heartbroken and in tears. We will miss him and his funny antics so much.
Thank you both for your advice and fast response. I wish I had better News