Seeing as is the generally accepted form; I dont recall ever hearing anyone say seen as (though its possible I just didnt notice). Grammatically speaking, seeing as is a conjunction; it fills the same function here as because. The Oxford English Dictionary defines it this way: seeing that, hence ellipt.
How do you use seeing as?
: since —used to introduce a statement that explains the reason for another statement Seeing as youve met my family, dont you think I should meet yours?
Is seeing as informal?
From Longman Dictionary of Contemporary English seeing as (how)informal (also seeing that) used before giving a reason for what you are saying I might as well do something useful, seeing as Im back, she said.
How do you say Im looking forward to seeing you?
To respond to a message that says “looking forward to seeing you,” we can simply say “see you soon” or “see you then.” What is a synonym for “looking forward to seeing you”? “Cant wait to see you” is a casual alternative, and “I am hoping to see you” is a neutrally formal one.