It takes a long time to get teens to open up. Never push, that makes them want to run the other direction. Never judge, just listen. In time he will open up more. But, there will always be things they won't talk to you about.
This is a normal thing with teens and parents, probably more so when it's an opposite sex child. I don't think you can "make" him open up to you. Be willing to listen (not talk at him so much, but listen) when he does want to talk. See if there's an older boy or a man in your life who would be willing to be a guy friend for your son and be available to talk about those "guy things" that boys need to talk about to someone other than mom.
I am 15 years old and a great relationship with my and my mom. So what he does what I think is really cool is that he does things with me that I like. He doesn't drag me to meaningless places that"build character". He plays ping pong, takes me to movies, jokes with me and helps with my homework. In turn I tell him everything about my day during these activities. With my mom I always told her everything because the car rides to my middle school took 20 minutes. She makes me put my phone away and we in turn ask each other about our days. My point is build a kind of friendly relationship that he can respect and that you can respect. If you need to be harsh be harsh. I know that when I am not too motivated my parents they always push me by taking my computer away at 10. So just try to build a trust relationship with him. Tell him about yourself so its not just you asking him about his day and his personal issues but also tell issues about yourself. My dad rants to me about everything and so does my mom because we trust each other.
I am 14, you probably do not think that I know much, but I found that when my mom told me stories about her experiences that she wanted to get me to talk about, I got a little more comfortable. If it something that your not worried about then I wouldn't worry about it, he will come to you sooner or later if he needs help. However, if it is urgent and you still cannot get him to open up to you, I recommend getting a brother or father or another trusted male figure to talk to him about it and help him figure things out with him. Hope this helps
I am also a single parent. I have a son that is 15 and a daughter who is 6. What you need to realize is that being a single parent affects our children. A combination of their personality and what they're dealing with will dictate their attitude towards you and life in general. Let me tell you, there is no quick fix. There isn't one answer or solution that will bring your son around. You need to put yourself in his place. Boys are different from girls. They process things differently. You need to do some research. Go to a therapist. Go to a family therapist by yourself. Then after some coaching see of he will join you in therapy. Raising kids by yourself is not easy. It never has been, and it never will be. If you love your kids you will make the effort and do the research that is needed to raise your kids right.