A real-life Grinch was sentenced this week to 18 months in jail for his role in stealing a boy’s Christmas gifts from the front porch of a Jamaica Plain home last Christmas Eve, the Suffolk district attorney’s office said. Ernest Polite, 40, of Hyde Park was sentenced after being convicted by a jury Tuesday on charges of trespassing and receiving stolen property, prosecutors said Thursday. Polite was already on probation for a drug trafficking case in Maine and was convicted on drug distribution charges in Dorchester in 2006.