Hi, Heidi,
What time of day will your son arrive? Which terminal will he arrive in? I´m asking because there is a bus from T4 at 12:15 (Mon-Sat.), which winds up going back into Madrid to the Av. Américas station but he wouldn´t have to change. As you can see on the link provided above, there are other buses, but he would have to go into the city to catch them. If he arrives in any terminal other than T4, he will have to get a bus from that terminal to T4, but there are frequent intra-airport buses. Iberia and its partners (One World) all arrive in T4; anyone else arrives in T1,2, or 3.
If the 12:15 bus won´t work and he has to go into the city, I would definitely recommend the Avda. Américas station over the Estación Sur station (which is far from the airport). There is a municipal bus, 200, which stops at all airport terminals and will drop him off at the Avda. Américas station. Some information here:
http://www.aena.es/csee/Satellite/A...554326802/City-Buses.html?other=1237554327072
Hope you have a wonderful camino, Laurie