Sorry to look as if I am trying to contradict everyone else who has posted on here but...
Wait until as late as possible to get this done - a decent dentist will get a mouthguard done within a week, so take impressions just before term starts and you are likely to be fine. Schools tend not to start contact sport until all the kids have their guards. As children grow most over the summer you will max out the usefulness of the guard by waiting.
DH says it is very rare for a child to need more than one mouthguard for an academic year, so yes your son ought to know when it is getting uncomfortable and that probably won't be until at least Easter (when there won't be any rugby anyway). Your dentist, if they have fitted it, will advise you if, as KB says, you pop in. Buy one through school and you don't have that option (and they aren't any cheaper either).
It won't surprise you to know that he wouldn't allow any of ours to wear a boil in the bag one. 'You get what you pay for' is his only comment on that.