Can you identify exactly what you'd like to know?

Responsibility for the baby's death? The autopsy report will indicate a cause of death. SIDS is listed when the medical examiner has been able to eliminate all other causes (including toxins, drugs, or poisons in the baby's system). Do you know if there is any evidence that bloodwork taken at the time of birth indicated the presence of drugs?

It's correct that DCF doesn't get involved in legal possession of a deceased. And a hospital is going to turn the remains over to the legal parent (next of kin). Since your brother created this child without the legal protections of marriage, including the usually automated process whereby he would have been assumed to be the father, he's probably out of luck.

Is there some legal reason that he wants paternity to be legally established, or is it just for his own peace of mind that the child was really his?
Legal access to a body is different than a court simply ordering a blood test on a live infant.