And I can understand why you would be concerned. My youngest son will be 3 in 3 months, is the thinnest of all my boys, and he weighs 32 lbs. My first thought is, how tall is he? My boys are average or above average in height so while my 6 1/2 year old and 9 1/2 year old weight 71 and 104 respectively, they are not overweight because of their height. (My 9 1/2 year old looks like he's at least 11 or 12.)
What have the doctors said? There is nothing physical to explain why he won't eat? If so, at 7 years old, I would be pushing him big time. Right now my 2 1/2 year old is my pickiest eater of all my children and I let him be because he is only 2 1/2 though I do still "try" at every meal. The minute I feel like he is at a better stage for communication (perhaps 3?), I will absolutely begin pushing him more.
At 7 years old, your son needs to understand that his health and growth are affected by how he eats. He cannot survive on a liquid diet and if he looks too thin, I would not be surprised if people don't start commenting or making calls wondering if it is something you are doing (obviously them not knowing the circumstances). *I* know he is the one choosing not to eat. *I* know you are a concerned, good parent. People who don't know those things may wonder though and I'd hate to see your family put through something like that. :-|
At 7, he needs to understand what healthy eating is, why he needs to eat 3 square meals a day, etc. Tell him what it could do to him! If it scares him, so be it! My kids hate shots to the point my oldest has had to be held down. Too bad. They aren't getting shots for me to be mean. They are getting them to protect them. And I tell them this. I tell them that if they don't get the flu shot, with them having asthma, it could end them up in the hospital with straws (IVs) and blood tests or worse, dead, and that both could be avoided by one quick stick of the flu shot. They get that and they understand the alternative is far worse so even though I may still have to hold them down, they get their shots.
Get a fun kid cookbook and have him go through it with you and find things he would like to try. Whatever you need to do, you need to get him eating more. I'm not saying turn into a glutton but SOMETHING healthy for at least 3 meals a day. Even if he has some cereal for breakfast, a sandwich of some kind for lunch with some fruit, and then a healthy dinner...then I'd add things like fruits and veggies, yogurt, or V8 Fusion.
I really hope you know I wasn't saying anything with the thought that you are to blame for the situation. I know how head strong our children can be but this is his health and you obviously recognize his lack of eating is a BIG problem. This doesn't fall under the "oh they won't starve themselves" category in my opinion. To me, this is no different than if I had an anorexic teen!!! Deal with it the same way. In fact, maybe that is the route to take! If you've already covered the medical side and they say nothing is wrong, take him to an eating disorder facility. Ask them for help via counceling with him if you can't get him to change his behaviors for you yourself. **Let him see the people who are sick with eating disorders, how thin and unhealthy they are, and what they are going through!!** Maybe seeing them will help him to see the path he is on because in all honesty, at what age would refusal to eat finally fall under an anorexic diagnosis? I firmly believe that SEEING a possible result of a path they are on will help them to see you aren't just making stuff up to get him to do what you want. Let him see the concern is real and founded and the end result is NOT good.
I really hope you let us know how it goes and what ends up working. I feel for you, I really do, and like I said, my youngest doesn't like to eat either unless it's yogurt or cereal some days. I may just find myself in the same boat as you in a few years so I really hope you will let us know how things are.
((hugs))