A Waltham man died Saturday after the car he was riding in rolled over into a tree off the F.E. Everett Turnpike in Merrimack, N.H., authorities said. Matthew Caban, 24, was riding northbound around 12:30 p.m. in a 2005 Buick LeSabre driven by John Caban, 92, of Waltham, when the car drifted off the road, according to New Hampshire State Police. The younger Caban was taken to St. Joseph Hospital in Nashua, where he was pronounced dead. The older Caban was also transported to St. Joseph Hospital, but with non-life threatening injuries.