It all depends on your point of view. If it's money, then it's a promotion. If its a chance to coach at big-time college football, it's a promotion. If it's a realistic chance to make a long-term mark on a program, or keeping your family in one spot for a long period of time, it's probably not a promotion. While you have almost no chance of moving from a D3 to a D1 head coaching position, he will now get the chance to climb the ladder and have that rare as hen's teeth shot going this route. But usually it involves a lot of hopping around, every couple years, long hours, miserable pressure, and good odds of getting canned because a whole team under performed, whether it was your position or not.
Really it just depends on what your long term goals are. I wish him a lot of luck, it's a very hard path to climb at that level.