What are my chances of gaining custody of my 4yr old son? His mom has full physical custody and we have joint legal custody.

mom has a long history of mental illness and has been place on 72 hr. holds twice in the last year. mom has borderline persoality disorder with a history of self mutilation, eating disorder, and severe depression. I myself have none of these issues and no criminal record. Our son is well taken care of by her, however I am deeply scared that could all change any given day. mom does provide basic food, shelter, clothing. but her own personal health and safety may jepordize our childs.