Have you spoken to your parents about it. Having been dogsitting, maybe now is your best opportunity to say something like 'he seemed to be in a lot of pain and not happy, have you taken him to the vets for a check up lately?'
If he's having to pee little and often, it could be that he's also going into renal failure - or he may just have a treatable blockage.
Personally, I wouldn't like to keep an elderly dog alive if it's back end was gone and it couldn't take itself to the toilet, especially if it was in pain as well.
Whatever happens, keep your eye on him and if it's really obvious that he's suffering, get the RSPCA in - you will remain anonymous.
Skimmer - you might like to get hold of some of Andrew Linzey's books - he's tops on animal theology.