Housing benefit and grown up kids?

I have a 19 year old son who works at MacDonalds and earns a really rubbish wage because there aren't many shifts for him, he sleeps at his girlfriends house most of the time and comes home to sleep maybe once a week. Will this assist me in my housing benefit as they assume he's home all the time and therefore the amount i get paid is lower. He doesn't pay me any rent because he can't afford it and isn't here much. Thanks