Just a quick update. He seems to have recovered well and is back for a 2-week check upon Monday. He has had the runs a few times, even last night, despite finishing meds last week. I will mention it when I go back.
It turned out that he needed quite a lot of teeth out, including all 4 canines. One that was whole last year was sheared to the root. Jane said sometimes cats misjudge a jump and hit their chin, which can result in the canines smashing against each other and breaking. The odd thing is, she said it must have happened months ago as there was a tartar build upon them. I don't remember him coming in at any time seeming to be in pain or distressed, so no idea when it happened. He had quite a few rotten teeth too.