Middle of a divorce and I want jt custody son. soon 2 B ex has 2 children another marriage w/ full custody. chances of 50/50

I'm a father in the middle of a divorce where my soon to be ex-wife refuses at this point to give me any custody of my 4 year son since she only works misc temperary jobs and I've supported everyone for the past 5 years. She fears that she'll have to get a job and her child support payments will not be enough if she allows jt custody of my son. Since she already has full custody of her 2 other children from another marriage I'm being told that Michigan law is going to allow her full custody of my 4 year old since she's already the primary care giver of 2 other children. My son is my whole life what can I do to get at least 50/50 custody. I can't imagine being a weekend Dad only!!!