Long story short:

Going through a divorce.
DNA test ordered for both kids ages 3 and 4 months, both children not husbands (which hes been aware of)
He was only steady in the 3yr for the first year until he was told she wasn't his. After that he was neglectful and only saw her when it was convenient to him (almost never) He has had absolutley nothing to do with the youngest.

He wants full custody of the 3 year old and nothing to do with the youngest. What rights does he have?

I have called local lawyers and got everything from he has 0 rights to full rights as a legal father.

I think you are going to need to give everyone a little more info so they can help.
Were the children born during the marriage?
Is he on their birth certificates?
You called him the "legal father" so I would assume that the answer to these questions are yes. Courts look at fathers in two types, legal and biological. If your husband is considered the legal father of these children then I would say that he has rights to both of them even if he only is interested in the 3 year old.
Do you have a lawyer for this divorce? If not get one.