Hi Jessica! Wow, your son is one lucky young man to be able to have such special senior pictures. Using
Epcot as a background will make amazing shots for his special day.
You did not mention where you would be coming from and will assume you will be on Disney property. If you are staying at the Boardwalk, Yacht Club or Beach Club resorts you can take the short walk to the International Gateway entrance. Ask cast members and they will head you in the right direction but it is very easy.
If you are coming from another resort I strongly suggest you consider taking a taxi or driving yourself if you have a car. Go to the Beach Club resort as that will be the shortest walk. Let the guard at the gate know you are there to meet a Disney photographer. You will either be able to park yourself or use the valet service but that is $25 a day. Again then walk to the International Gateway entrance.
If Epcot has early morning hours you could take the bus to Epcot from your resort and take the fairly long walk through the park and into the World Showcase to the International Gateway. Again cast members would be there to help show you the way. My only concern about this suggestion is that it is a long hike and if it is warm your son will not want to be sweating by the time he is ready for his photographs.
Have a great time with the photo shoot and cherish such special moments with your son!