Advice for my boyfriend

My boyfriend has a son from a previous relationship and is wanting to get custody of his son since the mothers been known/caught leaving him and her daughter from another guy for days with her mother with no way for anyone to contract her and last year he missed alot of school cause of her there's more I've just never pushed him to tell me as that's between them on how to raise their son. They don't have a legal custody order in place right now just an agreement between the two of them when they each get him. What are some things he should do to help his case when he takes her to court? What are they chance he could win in court?