As you read this response, please bear in mind that this is a public forum on a website, and that this answer does not create an attorney-client relationship, nor is it legal advice; it is simply an answer to a question asked in a public forum. Because it is a public forum, nothing said on this site is confidential or covered by privilege. As with anything in the law, there are exceptions to every rule, and you need to consult an attorney in your area for the best answer. Also, because of regional differences in practice, even within a state, what works in one area may not work in another.
Because DSS is only required to have "reasonable cause to believe" that abuse happened for it to "support" an allegation but a judge needs to have "clear and convincing evidence" that a parent is unfit and that it would be in the child's best interests before s/he can terminate parental rights might explain the discrepancy of the two responses. If you have proof of the abuse and of visitation with his father being harmful to your son, you might be able to get a judge to terminate his father's parental rights but courts are very reluctant to terminate the rights of a non-custodial parent even in the situation you describe.
You should consult with a Massachusetts lawyer who concentrates in children's and family law.
It sounds as though your son could really benefit from therapy. Courts are hesistant without clear proof that visitation is harmful to a child to shut that parent out of the child's life. As mentioned in the previous answer, the standard of proof is different with DCF and the Probate Court.
In addition to therapy (which may help with the proof and help your son deal with what has happened in his life), you really should consult with a lawyer about what additional steps need to be taken if what you want is for the father to not have visits.
I do not have the entire picture so it is impossible to provide specific advice for your situation. There are laws to protect victimized children from seeing the abusive parent, but these generally come into play after the abuse has been established and proven in court. I agree with my sister attorney that therapy for your son would be helpful to him. In additon, the therapist's testimony before a judge could influence a judge to rule that it is not in the best interests of your son to visit with his father. You might also consider "pushing" DCF a little on this issue and have them take a stronger stand to protect your son. Judges in Probate Court do take into consideration the views of DCF.