Your dog is an indoor dog? He does not go outside to bathroom or for walks?
If he goes outside at all I wonder if he is eating other creatures' poop? If it was at all possible he was eating cat poop that would be a place he could get the tapeworms. Or dog poop but I'm thinking you would be much more likely to notice him eating dog poop.
Sorry, not to be rude but, are you sure these are tapeworms? Really, we've had people post before with very strange and wrong things told to them by their Vet. I'm not sure if you can get Drontal for dogs that does not have Praziquantel in it but check the ingredient list on the container. Meds that work on most other worms do not work on tapeworms. It must have Praziquantel in it.
I recommend Barking Dog's father's treatment. A slug of strong booze. For YOU, to help you deal.