A Long Island man went to pick up his 13-month-old son from his estranged girlfriend for a visit yesterday — and stabbed the woman to death, police said.

James Schultz, 31, then drove the child from the St. James slay scene to his parents’ home in East Northport, walked into another room and fatally shot himself.

The body of Schultz’s ex-girlfriend, Katherine Farrell, 23, was discovered after detectives couldn’t reach her when she failed to show up for work at a nursing home.

There was no record of domestic disturbances or police calls involving the couple.