I seriously doubt it was intentional.
I have been to Egypt more than once (I just got back in November, and I am going back in a few months for another visit), and food is a big deal to Egyptians. Any family with enough money to host an exchange student would have enough money to feed him, and would feed him, since that's part of being a good host or hostess.
My guess is that he didn't like the food. Most of it is delicious, but there are some things which folks from the US might not want to try. I know there are certainly things which my dad's cook fixes which I just won't eat because I don't like it. If he wasn't prepared in advance for the different diet, he may have not eaten much, which would account for his weight loss. It's also possible that he didn't get all the proper vaccinations, and that he had severe stomach problems while there which made it hard for him to eat.
Whatever the cause, knowing how much Egyptians value being good to their guests, I am betting it was not intentional at all.
**EDIT**
Someone mentioned that Egyptians are selfish with their culture. That is not the case at all. I have been all over the place, and lived in various countries for extended periods of time--Egypt was one of those countries. The Egyptians are not selfish at all, and hospitality is so important to them that they would never willingly let a guest starve. They are generous to a fault, so that information is inaccurate.
**SECOND EDIT**
After reading the article in the link, I can see where the problem came in. The Copts do fast a lot. And unlike the Muslims, they may very well have expected him to fast along with them. I have been in Egypt during Ramadan, when they don't eat during the day, and I was still fed to bursting because Muslims consider their fasting to be a personal thing, and don't expect others to fast as well, just because it's Ramadan. They'll fix you food that they cannot eat until later, just to make sure you don't go hungry. It's too bad he went to a strict Coptic household. He would have been better off with a Muslim family.