The good news, this is completely common for kids who have experienced trauma. They want to control everything, and no one can force you to eat. Since it is common, there are theraputic ways to help. You figured out the bad news, it is obviously unhealthy. It sounds serious enough that I would discuss a specialized therapist with the social worker.
Another thing (he is eight, so hopefully he is somewhat communicative) is that familiar food can be comfort. Do you have any sense that along with all of the other changes, they type of food you are offering him is a drastic change from what he was used to? If he was used to a diet of fast food or junk food, or food of a different ethnicity, he may just be having a hard time. Obviously, you want to keep your foster son healthy, but you may want to consider a sort of "transition diet."
Lauren- He's been with my husband an I since the week before easter. Since we've brought him and his brother home, his brother has been removed from our home due to physical threats to me. So that's another issue we've been dealing with, with this little guy. He seems to think, well if I'm naughty they will kick me out too, which we've told him isn't the case and even last night he brought up the infant adoption program were going through right now and asked if we were going to adop him but 99% of the time he hates us and our home! He' so up and I wish I could fix it for him, hesa a skinny little guy and I wish he would just put somemeat on his bones.
We had a 4 year old foster dd who had a issues around eating and would often not eat very much at first. She was an emergency placement with us, we were 19 & 21 years old and had NO IDEA what we were doing and no support or training from DCF, so we were just kind of winging it. There are so many things I wish we had done differently when I look back but anyway... She transitioned pretty well, had hardly any behavioral issues with us, and her daycare was amazed at how she quickly became much happier and had better behavior once she lived with us, but it seemed like all her anxieties came out in how she ate. I think 2 issues were at play for her. 1) Our food/eating habits were not at all what she was used to, and 2) She had too much anxiety /stress to eat, even though she didn't show it in other ways. We all know the feeling of being too stressed out to eat, I think she was experiencing that. Also so little in her life was under her control, I understand using her food as a means of controlling her life at times.
For the second thing there wasn't much we could do other than try to help her adjust and so on, for the first thing we basically just fed her what she was used to. We had never, ever gone to Burger King, we started bringing home kids meals for her all the time. Everything had changed so much for her, it was better to feed her unhealthy food and have at least that little bit of consistency. Also she had been used to cooking her own food, using the stove herself, etc, and she actually really loved to cook, so we started doing a lot of cooking together, & involving her in choices of what we were going to make. She was used to just getting food for herself whenever she felt like it not having meal times. We would all sit down to dinner together, we would offer her each food but if she didn't want to eat (she pretty much never did at first) we wouldn't comment on it, we would just save her food and offer it to her again every half-hour or so. Gradually we transitioned to eating meal times, but at first we would just focus on sitting together, then if she didn't eat we would let her eat later and not make a big deal out of it. We did "mainstream kid" type meals, she pretty much would only eat hot dogs, pizza, mac and cheese, happy meals and everything with chocolate milk to drink. She only ate strawberries as a fruit and carrots with ranch dressing as a veggie so that's what she ate at every meal. As she got more used to us and our food we gradually upgraded to healthy versions, like Annie's whole wheat Mac and Cheese, organic turkey dogs, pizza with veggies, and so on.
It sounds like 2 very different kids but maybe this will help a little. Good luck!