Nursing homes might be a place where your son could get some medicine-adjacent hands on experience. Meal assistants/servers type jobs are open to those old enough to legally work. In MA, he can get a work permit at 15.
Many nursing home will pay for CNA training for people who are already working for the facility in other capacities.
Another idea is hospice volunteer. He wouldn’t be able to do any medical procedures, but he could spend time with the patients in hospice facilities and get a first hand look at the process of dying–which is actually a pretty big part of medicine. Every doctor has patients die, even doctors in the so-called “happy” specialties like pediatrics or orthopedics. iI 's part of being a doctor.