I know that I wondered about this question myself, early in senior year. My son is on the young side for his grade, too. Then someone (actually, a coach) told me that they mature tremendously in the senior year and just seem to realize that it is time. As it turns out, he was right as to my son and he is so ready for college.
But there are many kids who do take time off, and there are also lots of programs, from private school postgrad years to overseas programs to Peace Corps type programs; and then there are kids who just work.
You might want to research some of the programs through your school's college/career advisor, if there is one, or on the internet, and have a backup plan. Local junior college is also a good option if the kids aren't ready. They can still play ball, and take as light or as heavy a load as they want, and still be living at home and transitioning more gradually to adulthood. Several boys on my son's high school team are doing that, partly for academic reasons, partly for playing time reasons, and some because their parents or they felt that they were not ready to leave home.
I guess if it were my son, I would lightly broach the subject and see if it seems to appeal to him a lot. I think, though, that I would still encourage him to apply because things might change. A lot of colleges allow kids to defer their entrance for a year. It would be tougher if he committed to a coach to play, though, and then wanted to take a year off.