No, grandparents are not his sponsor family. My DS decided that he would like to experience an assigned sponsor family, just like the other cadets..and hopes for a good match.
My C3C also wanted to have a "regular" sponsor family even though his grandparents live in the area. However, there is a limited number of sponsors and even though he did get assigned one, he never met them because he took the opportunity he had on their free weekends to visit the grandparents. Luckily my parents have plenty of room and took along whatever buddies wanted to go along. My parents eventually signed up for the sponsor program so they could get easier access to the campus since they are not military and my son gave up his other sponsor so another cadet could be assigned.
The grandparents have been a great benefit to me and it is wonderful knowing that if I can not attend an event, the grandparents are there to fill in. They went to Acceptance Day last year and even went out to cheer the cadets on their way to Jack's Valley. They bought season tickets for football and even though they weren't able to take him out every weekend, he was able to visit with them during most games. It has been nice for me that I could talk to someone who has actually seen him since my DS is not very communicative on the phone. And thankfully, they have been a reliable ride to and from the Denver airport!
My DS probably only went to my parent's house for the weekend, maybe a half dozen times last year. They did come down and take him to dinner occasionally. He would go to church with them if he stayed over Saturday night and I think they came and went to the Cadet chapel with him once, but there just aren't that many free weekends. My DS and his buddies went skiing as often as they could during the winter so if it was a free weekend, they would head to the slopes on Saturday and had to catch up with studies on Sunday.
He just returned to COS for third summer session and told one of our neighbors he was going "home". It has been an incredible year for him and he seems to really love it. This first year has flown by and I am sure the next 3 years will go even faster.
TMalles - your DS will work out whatever schedule he needs to be successful and he will do great. This forum is great for advice, but there is not a "one size fits all" rule on how to be successful so tell him to work out whatever time he wants to spend with the grandparents and let the grandparents know that he has the right to say he can't see them without them getting their feelings hurt. I had to let my dad know that he had to communicate with my DS about his schedule and not go thru me. Once they had that figured out, it has been great for both of them. Good luck to your son.