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.
Pediatrics Expert Advice from Henry Bernstein, M.D.