In my humble opinion: (1) confronting this person will not make him more willing to pay you your fair share of commissions because to do that will indicate to him you think he's a liar and a cheat; (2) taking him to court could possibly totally destroy the business relationship and the business you've built so far; (3) he's obviously stealing from you and you reciprocating will only cause more distrust between you and more confusion.
One idea would be to find someone to buy this dishonest person out or in the alternative see if a small business loan or a loan of some kind would allow you to buy the person out and you become a sole business entity. Another suggestion -- if you can -- this person will not get his money paid back if you aren't selling anything and making commission for him to rip off. Quit selling until you get paid fairly (that's hard to do but WILL work!) It's a fact -- if one's financed a business that isn't making any money to pay them back -- well, bad investments usually draw an investor's immediate interest and also his help in attempting to turn things around.