Another option you have, in the short term, is to make a notification to the DOCS. As a health professional, I would be obliged by law to make a notification given the use of drugs and subsequent neglect in the household. It is possible that if DOCS were to intervene, your ex-DF would be granted a temporary full custody until the mother either got her act together, or alternatively, decided to cede custody to the father.

Any member of the public can make a notification to DOCS. From your story, it sounds to me as if the child's needs are not being met in the mother's household, and for that reason alone, I would urge you, or your ex-DF, to notify DOCS so that they can intervene and at the very least provide the mother with support in controlling her substance abuse and becoming a better provider for her son.