How do I get started in my son’s life, he is 10 years old and I will meet him for the first time.?

I would definitely get involved in counseling for him as well as your self both individually as well as together. A good counselor will do that however. Simply, be yourself, introduce yourself. Answer his questions being very careful not to put his mother down (even though she is wrong) she is his mother and this will only cause you as well as him great pain. Ask him about his interests, sports, school, etc. I bet you will find that the two of you have a great deal of common interests. Tell him about the other side of his family that he has no idea or knowledge of. His grandmother, grandfather, any other siblings, aunts, uncles, cousins, etc. Just be yourself being very careful to bite your tongue and not speak ill will about his “mother”. However, being honest in a very gentle manner. The big question he will most likely want the answer to is why you have not been in his life. Tell him it was his mothers choice and you had no control but you are sorry. He would need to get his mothers reasoning from her. Good luck.

First the person then the position. Allow him to get to know you as you get to know him before you try to connect on the father-son level.
It should be his choice what he calls his mother’s boyfriend and what he calls you. I think you can appreciate what a dilemma this must be for him.
By all means, accept the counseling.
Ten-year-old guys need friends, not complications or tug-of-wars.
Best wishes,