
Hi Guest,
In part it could be due to how late you are feeding him too ;) By 5 months most, but certainly not all, puppies have reasonable control of their bladder and bowels - like humans they are all different though. It could be that the time scale you're leaving him for is just too long for him - almost 9 hours overnight. Perhaps you could stay up an extra half hour at night and get up an extra half hour in the morning and see if it makes any difference?
I always leave youngsters a maximum of 7 hours overnight (not literally left though, they share the bedroom with us :P ) and gradually extend it as they become more reliable.
It's probably worth trying to ascertain WHEN he's actually soiling his cage - since it could be he does it almost as soon as you leave him and is actually caused through anxiety at being left. Why not join - then you can answer members' queries and with more info from you others will be able to provide more detailed help.
Regards, Teri :)