just ask. that's it.
and if he says no...

you could either:
1.) go scarface on him - "@#%$!!!!!!!!!!"
OR
2.) ask mom. "moooom... dad won't let me blah blah blah"
OR
3.) bribe him? "i'll tell mom about the truth about your 'business trips'

"
OR
4.) reason with him. "i promise i'll clean the pool, wash the dishes, etc etc etc"
OR
5.) elope "i hate you guys! *runs away*"
OR
6.)
LIE "i'm spending the night at mary-sue's house, mmk?"
...decisions, decisions