Hello and Thank You for using Just Answer. Just to give you a heads up, 15 year olds boys can be very moody. They can be the nicest and loving child one day and then mad at the world the next day. They also think they know everything about anything and are not afraid to challenge your judgment.
To assist you in dealing with the 15 year old, first of all get rid fear. If the boy senses that you are in any way fearful of him, he will walk all over you and try to manipulate you every chance he can get. Just be as loving and supportive as you can. Set boundaries along with his dad and be firm and consistent. Do not be afraid to enforce consequences. Do not talk down to him. Talk with him and to him. Also give him a chance to voice his opinions but the parental figure has the final say.
You will be fine. Just develop the courage and mental fortitude to spread your wings and be a great person in this boy's life.