The age at which students can take the EMT course varies. You have found the answer for your state. Once your son is 18, he can contact the local ambulance service, or the local community college. Many offer courses for EMT certification, and sometimes this can be done during the summer between HS senior year and college freshman year.
RE: CNA…you need to check that age requirement in your state. This also varies. But 8 think it’s often 18.
In some places you can start the courses when you are 17 but can’t take the exams until you are 18, or you need to have your 18th birthday before the course ends. So check.
Some places offer “cadets” programs with their first responders. This isn’t a certification but it is some experience with these first responders not providing services!