'laid back' is an expression in English meaning the person is calm, or easy to get along with. 'laid back' can also mean someone is laying down, like when you go to bed. In this case it sounds like they are making a play on words, saying he is very 'laid back' Since if he is horizontal, that means he is all the way laid back. Hopefully this helps.

Horizontal means being a prone position, laying down. He is so laid back that he sleeps all all the time.