'Sean' - I would still contact them - they need to know that something like this happened to one of their pups - maybe they can offer some advice and reasons why this may have happened. You are not having a go at them - just giving them information.
So you didn't get him de-sexed at 6 months - that is really no biggie - just say you wanted him to finish growing first - on advice you had received. You don't intend to breed with him - so all is good !
Yes - with the symptoms Mojo is showing - sounds like an infection tied in with an enlarged prostate. So yes - as the vet said - you will probably have to de-sex him sooner rather than later. But you need to get rid of the infection first.
Heaps of for you all and I really hope the vet visit on Thursday is not too traumatic for the hunk !