Custody dilemma...need help?

My boyfriend is having trouble seeing his daughter with his first baby mamma. He is supposed to have partial custody and he doesn't even get that. We have a daughter already and one on the way in February. He is wondering about how to go about getting custody. Either the partial custody that he was granted or full custody. What things do the courts look at when deciding who gets custody and what things hold you back from getting it? His other baby mom said that if he tells their daughter that she is going to have a sister then he will never get to see her again. He has been in trouble with the law and is working on things to stay out of trouble. He has never had a felony. What things on his rap sheet would keep him from winning this battle. Im looking for any information that could help. We live in MN. They were never married. Me and him both are going to start working on this project to get him the rights he deserves. Feel free to ask any other questions