6 year old boy with bulimia?

Hello. We have been approached about fostering two boys (ages 5 and 4), and the oldest has problems with throwing up/bulimia. Today, we were part of a panel of foster parents who spoke with those who are completing IMPACT foster parent training. One of the other panel members just got two boys, and one of them has bulimia. When we arrived, they were having a lunch, and the boys were shoving a lot of food in their mouths very quickly. My partner and I went out in the hallway area to let the kids play a bit, and the oldest was sitting on a couch eating a cupcake. He stuffed so much in his mouth! As I was telling him to slow down and be careful not to choke, his gag reflex set in. He ran to the bathroom and threw up in the toilet. He kept throwing up until there was nothing left. Apparently, this is is quite common. I stood there with him, and he kept saying, "mommy be here in a minute?" It was pitiful. It seems like both children have had to eat as much as they could as quickly as they could in the household from which they were taken. I wonder if he has Pradder Willie syndrome or some food allergy. Certainly, they could do a genetic test for P.W. Also, the oldest has never been to school! The 4 year old can't talk yet. Their foster parent's grand-daughter told us that he will drink something and make his stomach flex in a way that causes him to throw up. Has anyone else dealt with this? Do you have any idea what might be the cause (is it physiological or psychological)? Thank you for your replies. :)