Well, I have a couple of suggestions with the information you are given me as I understand it.
First. If possible he could go with his friends to San juan and stay overnight in a hotel closed to the airport (Isla Verde) there are several inexpensive hotels in that area just few mimunes from the airport, once he is there he could take the bus (public transportation) that goes to the airport (around $1) or take a taxi from the hotel to the airport (around $20).
Let me know if that is feasible before telling you plan B.
As a summary this is how I look at it:
1.Friends need to go the night before his flight, lets all go together to San Juan.
2. Friends depart and your son can stay in a hotel overnight. I recommend staying in the airport area (Isla Verde) for easy access to the airport.
3. Next day your son can take the bus or a taxi to the airport wich is few minutes away.
Let me know if this idea works for you.