If your DS really wants to enroll at UMich then let them know--not just that he received the scholarship, but that he really wants to enroll there.
Our DS was accepted early decision to UMich in Jan '12 and wanted it badly. Received 4yr NROTC scholarship in Feb. Unfortunately, he was DQ'd for an eye injury. The fact that he was out of state probably made little difference. We travelled at a considerable expense to visit the school and the NROTC unit. The only person in the city of Ann Arbor who did not give us the DMV treatment was the administrative assistant at the NROTC unit, who was first class.
DS's best buddy from high school applied regular admission to UMich and was denied. He was, however, accepted to Columbia, Princeton, etc. A good friend who is in the test prep/admissions advising business told me that the kid was denied because UMich admissions didn't believe he would actually enroll. Admissions departments strive ultimately for the high yield, i.e. the greatest number of acceptances that turn into enrollments. Qualified applicants are a dime a dozen.
What I can tell you from our experience is that UMich couldn't care less about any individual student. I would never discourage your from letting them know that your DS has a scholarship in hand. But you should know that there are probably five thousand kids, from around the globe, whose parents are ready to pay full freight+ for their kids to enter UMich or dump UMich if they get into Stanford.
If DS really wants UMich then let them know.
Best of luck!