Hi Elston,
The claimant is entitled to a written appointment 'invitation' and should be given at least 7 days notice ( this includes the time the letter takes to arrive via the post). They do not have to attend anything offered over the phone or at short notice.
However, from the fact that your son received a reminder, it looks as if the Maximus think a letter has been sent.
You can certainly ask to have this changed. The claimant is allowed one chance to rearrange without their claim being at risk, the second appointment must be attended. You can try and argue that this should be rearranged without penalty as no letter was received - but you might not have ,much luck with that argument!
It is worth booking the next appointment while you are on the phone cancelling the first. This means that you will have a bit more choice of date and venue.
Nothing on this board constitutes legal advice - always consult a professional about specific problems