Father rights to get full custody in another state

Our son is divorced. There are three children. The oldest, a boy (from a previous relationship), was adopted by our son. The mother has custody of him through school year; father has custody of middle child and the youngest girl is back and forth between mother and father every other month. The mother is unfit to have the children. Bi-polar, supposedly drawing social security because of a eating disorder and high blood pressure. She hasn't held a job for a long time. When the kids are with her, they are up too late, don't eat right, have gotten very mean, especially to each other. They have witnessed her raging at our son, and at them. They cower when you get close, they are afraid of everything. Our son and the kids have been living with us in Iowa, we purchased a business and he is a big part of it. The children have become well adjusted here. They go to a very good day care provider.
My question is, what does my son have to do to get full custody. His x-wife keeps telling him that she can make him move back to Michigan because of the children if he makes any waves. She threatens to take him to court quite frequently. In my opinion, with her track record I wish she would try. She is on probation for possession of a controlled substance. I believe that ends in March. Anyway, please advise.
Thank you in advance.