You are experiencing what lots of other families already have. As you may know, more than three fourths of all kids are in some child care setting and so this is very much a fact of life. This has been increasing over the years. The many kids who attend child care share their bodily fluids (saliva, for example) and tend to have increased exposure to germs. This results in some kids ending up with more illness than if they were home with just their parents or sibling (less exposure). Prior to child care, kids would experience more of these illnesses at home with their older siblings or have increased exposure upon entering kindergarten.

There is nothing out of the ordinary that is recommended to build your child's immune system. What you have described doesn't automatically mean your grandson's immune system is any different that another similarly aged child. It sounds like your grandson is as healthy as the next child. Simple hygiene maneuvers such as good, frequent handwashing and keeping children who have febrile illnesses home until they are no longer contagious may make a difference. I would not want you to necessarily take your child out of child care as I assume your family feels the benefits outweigh the negatives.