Be honest. Explain it like he's a 3 year old.
Lesa, Jane, my wife is a teachers aid and she works with pre-K children. She is going to ask the teacher she works with about your question and she will get back with you. This is a difficult time for every member of the family including Wesley. I have always said that when a member of a family joins the military the whole family joins. Hang in there.
Many 3 year olds (as you may know) are vacuums for imformation. I think the visual images of actually seeing the planes, uniforms and even some things he could actually put his hands, like a toy airplane, would help the most. Is there a AFB near you? If so, take him to a yearly Airfest that they might have. Like MacDill AFB in Tampa, Fl has. Or sometimes the local Airports have them. There, he might be able to touch them and go inside some of them. And, after Wesley is in the AF, have him send back pictures of him with the planes, his friends and such. That will help put the images all together in his mind.