The vet supplied steroid and antibiotic injections as a catch-all measure saying the cause could be all manner of things - this was on the telephone, not an examination.
The boar has been on his feet during today and did a littler wallowing and lying out in the sun. He has also eaten small quantities of prepared food laced with banana and other tasties.
Thanks for your input Fleecewife:
but you can palpate his jaws from the outside gently and see if he reacts to pressure.
I did do that and he did not react.
The wider issue here is how best to manage the end of his life. When he stopped working we decided to keep him into retirement - yes I will admit it - for sentimental reasons. We always said that while he had a life where we could do all his natural behaviours we would look after him but we would know when time was up. This little episode is making us address more clearly exactly what indicates that time...