Sex Talk with 6 year old

Ok here is my situation. I have a 6 year old boy, and we were over at a neighbors house the other day. My neighbors son is 5. I also have another son who is 3. All 3 boys are usually loud and playing everywhere in the house. So when it got quiet in the room, I became suspicious. I went into the room and found my youngest watching a movie on the bed, but the other two were no where to be seen. The bathroom door was shut and the light was on. I had my neighbor open the door to the bathroom. She found her son laying on the floor with his pants and underwear down and my son was sitting beside him. We talked with both boys and finally got out of them that they were "playing sex" (After my 3 year old had told us). We asked each boy if their mouths had been on the other's private parts. & They said yes. So I went home and had a talk with my son, and my neighbor had a talk with her son.

Everything was fine and both boys seemed to understand that no one is allowed to see or touch their private areas unless it be a doctor or their parents. Well a few days later I found my oldest son on top of my youngest son (who had his underwear down) saying he was the dad and was kissing his belly. My youngest later told me that my oldest was licking his private area.

After the situation with the neighbor happened, my ex had a talk with my son. I then found out that he had set up an appointment with a psychologist for my oldest son. I'm not sure what to do. I've had 2 talks with my son and still this continues. Any advice??