Legal advice - how to keep a house when separating?

My boyfriend doesnt have credit to buy a house his family member and i signed the mortgage together and my boyfriend put the down payment down. things arent working out...fighting all the time, his ex is saying theyre sleeping together and he lied and said he hasnt called her...and eventually admitted he did...and said he called her from a payphone to tell her to leave us alone...his excuse: "i didnt want her getting my number"...which hasnt changed since they were together...anyways is it possible for me to gain the house and him leave legally? would i have to pay him back the downpayment? how does it work when you own the house and someone signs the mortgage over? do i need to be reapproved? will i need another co-signer? or will they just see every mortgage payment comes out of my bank on time and clearly i can make payments?