How does a mother get legal custody?

Usually when marriage isn't involved mother is umed custodial parents until a court says otherwise. It is best though to have the paperwork anyways given your situation. He would have to prove he is capable of providing proper care and you two would have to be on good terms for 50/50 judges rarely grant it if parents are fighting anymore because it can be way to hard on the child involved. Typical visitation is every other weekend and that is probably what he will get unless he can prove you unfit.