Need advice

So my husband of 8 in half years cheated on me with my best friend. He doesn't want to be together okay it is what it is. Second we do have 3 year old together, we want a divorce but the issues is I can't stay in New York I have no family but my son dad, I will like to move back home. Right now I came to Washington state for a month and I don't wanna go back I wanna start my life down here. What are steps I need to do to get the divorce process start and get full custody my son over here so he can't say I didn't steal him. I am agreeing with visitation and breaks and I'll pay for all that and I know he will too. But the thing is I know it's going get messy if I tell him we not coming back. He is going be angry and not think. So I need advice what I can do.