How do you get bowser to his kingdom in Bowsers inside story?

after awhile you can get out through the pipe spot.but first you have to help the moles with bowser

You get hit by a laser. then you fall down and the pipe yard inside bowser opens up and you get out through a pipe

Unfortunatly, unless part of the storyline, this doesnt happen.

keep hittin him with a hammer and special attacks

Yes, when his minions feed him too much food when he defeats Midbus.