How do I obtain full custody of my child in the state of Michigan?

My (ex)husband and i got married in 2008, had our son in 2009. In 2010 we moved here to Michigan, and we decided we werent fit to be together anymore. He moved and got his own apartment, and i later met someone and we got our own place. My husband and i havent gotten a divorce yet due to lack of funds, but that process is soon to begin on my end. Now with all of that being said, when my ex and i were living on our own he lived an eighth of a mile away from us and never once came to visit our son even with an open invitation. Our son used to go over to his apartment until one day i went over there and there were beer bottles and marijuana all over the place. I refused to leave our son there because of the atmosphere which resulted in him getting physical with me in front of our son. Since that day i have not trusted him enough to leave our son alone with him, but i have set up visitation time with him, and hes failed to follow through to see our son. I get absolutely ZERO financial help from my sons biological father because he lives off of military unemployment and now no longer has his own apartment, he lives with friends. I have many many MANY witnesses to my sons biological fathers strange unstable behavior, and unhealthy living atmosphere. I also have many witnesses to vouch on my end how hands on my boyfriend (who has been there everyday supporting and teaching and bonding with my son for the last year and a half) and i are with my son, and how we have created a stable living environment and home for my son. What would be the best advice you can give me on obtaining full custody of my son?