How can a 15 year old boy in America get imancipated from abusive parents?

Emancipation has little to do with the parents or the family relationships. The child needs to convince a court and a judge that the child is able, willing and capable of acting and functioning as a legal adult.

It is very unlikely until and unless the child has a job, finances, insurance and enough education to be dealt with as an adult.

If the child is being abused the only immediate answer is to contact the child protective service group for the area and request to be removed from the home.