Greg Mattison played his college football at University of Wisconsin-La Crosse (Division III). He played for arguably one of the great football college football coaches of all time, Roger Harring, who is in the College Football Hall of Fame.
Michigan's DC Greg Mattison played for Harring and the UW-LaCrosse Indians (now called the Eagles) in 1969 and 1970. These were Harring's first two years coaching at UWL finishing 5-5 in 1969 and 5-4-1 in 1970.
Harring never had a losing season and finished 261-75-7 (.771) 12th all time among college coaches and 4th among Division III.
I found this document from the UW-LaCrosse online library a very interesting read about Harring and Mattison. There are season summaries for Harring's football teams for 1969 and 1970 which include Mattison in the team photos. He wore No. 63 in 1969 and No. 66 in 1970.
1969 UW-LaCrosse Indians Team Photo: 8th row (top row) first player on the left is Mattison:
1970 UW-LaCrosse Indians football team: Mattison is in second row, 5th player from the left (No. 66):
Also, apparently Greg Mattison was drafted in 1971 by the Washington Redskins (see page 9).