Westbrook police charged a 19-year-old this morning with aggravated assault in connection with injuries to a five-month-old baby.

Police say Steven M. Waterman was baby-sitting his girlfriend’s infant son during the day Thursday. That night, the boy’s mother, Jamie Swan, 20, of Main Street, took the child to Maine Medical Center with serious injuries.

The infant is being treated in the special care unit, police said.

Waterman, who is not the boy’s father, is being held in Cumberland County Jail on felony assault charges.