Childs biological father is inconsistant

My son is four and his dad and I broke up when he was one. He started out taking him 2 nights a week. He was irresposible, like by not putting a coat on him in 14 degree weather. Out of desperation to have the father involved I always remined him about things I shouldn’t have to like to put a coat and socks on him. He did a better job and things were going well for about a month and a half. A month before my son turned two I started dating agian. I had a boyfriend we were serious.The biological father also started dating and stopped picking our son up, stopped calling, stopped answering the phone. My boyfriend and I soon moved in together. Then out of the blue when my son was 2 and a half after no call for six months. He called and wanted to take him to his family reunion. I told him ok but he needs to make more attemps to see him. He gave me a list of excuses like that his roomated didn’t keep the house clean enough and he girl friend was depresed and needed his comfort. and the lamest Sunday was his only day off to relax. Well I relax with our son and I worked full time at the time. I let him pick up my son, because I swore I would never keep them apart I dont want my son to grow up and im the enemy. Six more months went by with out another call. I even told him I would give him gas money if he needed it. He could take him to the park. He could even spend time with him at my house. By this time my boyfriend and I became engaged. He is a great man. He had been taking care of my son with me the past year and few months. diaper changes, tlc, insurance. meeting all his needs as a father. without any pressure from us my son started called my fiance daddy. Every couple of months or so the biological father would call. the last to january’s at tax time he called for his SSN number to take the tax credit- pathetic right? Every holiday to take him to his parents house who also never call or seem to be interested in my son. On halloween a couple months before he turned three he asked to take him in his costume but I hadn’t talked to him in three months. By this time my son was calling him Matt. and he said I know this is my fault blah. blah.

Then Thanksgiving and Christmas I let him go with him on Christmas last year. worried the whole time that my son was pretty much with a stranger worried that he missed me. Than agian no call untill tax time. This time in March. I moved two building down from him and he works 3 blocks away he had called and came by twice since then to bring my son fruit from his job. the last time was September than he called on Thanksgiving to ask what my plans were of course I had plans so He asked to take him to his dads house for Chrismas this year. Friday he stopped by for 20 mins and brought our son a birthday present, And asked him to call him daddy just once. so he said ok daddy, matt I like the present you got me.

All along my fiance is daddy. through love disipline everyday not just fun holidays. I feel like the bio father is disrupting our home with his inconsistancy. I also feel like he is using my son to show off to his family. He is still very irresponsible. I’m afraid if something was to happen to me that he would gain custody of my son. He smokes pot and has a criminal record due to a domesic incident with me witch is why we are not together,and no lisense anymore. I want to get married and my finance wants to adopt him. The only problem is that right now im in college and I’m afraid it will mess with my financial aid. I’d like to file for sole custody. I hurts me to think like that because I want the bio dad to be involved but I want struture and my son to know him better I am just so afraid if something happened to me that my son would be ripped from his whole family and everything he knows. My mom and dad and brother and sister are very very close to us. I doubt he would let my son see them.

WHAT CAN I DO??? What are my family’s and my fiance’s rights? What are my rights?