How do you get out of bowser in bowsers inside story?

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

You can walk over there, or you can take the purple swirly circle thing and press "Bowser Castle" and you'll be in Bowser's Castle.

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