How to train my lazy dog to be motivated?

I have a 5 month english bulldog and he is as lazy as can be. I sometimes have to carry him outside from the bed or he will take his sweet time to slowly waddle to the door, about 15 minutes if I don't pick him up. There is only a couple of steps in the house but I will be moving to an apartment in about a month or so and I will be on the third floor. I'm worried he will be too lazy/stubborn to make it outside to use the bathroom and get into bad habits of eliminating inside. Plus, he's a big boy. 40 lbs. at 5 months and growing fast. He is getting to heavy to be getting carried outside and in, as for I'm only 100 lbs. Any advice would be awesome!!!