Kathrine,
This post is going to be REALLY LONG... Hopefully you can get some ideas and will forgive me. ;-)
At 8 your brother should be able to put into words what bothers him about foods. Is it texture, taste, smell, chewing... find out where the "I don't like" comes from.
Then, get him started on a HUGE project of the Tongue. You can look at the muscles in it, the salivary glands, the little thing that ties it to the bottom of the mouth... See if you can get his teacher on board. If she's noticed he's thin, she'll likely be thrilled you're trying to do something about it, And probably let him do a classroom presentation. You can do a 3-D Model. Be as creative as you can. I can see using a body pillow and sewing tastebuds on it... Or, that 3-D floam and making a mouth... OR, even a wooden model... but you do whatever you think will get him excited. See if you can get a hold of a hand lense (magnifying glass) and let him look at a tastebud UP CLOSE and personal (probably YOURS...LOL, BUT! Cats' tastebuds are shaped like hooks! By the way...If you decide to look at the cat's tongue, I want to see the video! ;-).
Just do everything to learn what the tastebuds do and how...
Then, research what each area of the tongue does to help us taste...sour, butter, sweet, etc. Testing this will get him involved in experimenting the different areas. AND... get him to try all kinds of stuff. Have him sample some baby formula... he WONT like it. LOL... but his tastebuds used to think it was the greatest stuff ever!!
My girls would eat anything. Their babysitter would give me a free day of daycare if I'd bring them for meals she was introducing to the other kids, because she knew mine would dig in! There were several factors that made them this way.
1. I used to tell them that a sign of someone growing up is when they learn to teach themselves that other foods are okay. Only babies let their tiny little tongue tell them what they can and can't eat. You can explain to him that our tastebuds aren't really grownup until we are really, really old... at least 40 or so...LOL... and since they are always changing things will taste different to us as we grow... AND, we can train our tastebuds to like anything we want them to. Right now, his tastebuds are acting like little babies and not letting him fuel his body. He has to TAKE CHARGE of them and not let them boss him around, HE's the BOSS.
2. My girls grew up helping my husband's grandparents in the garden. They loved to eat what they grew. You could help your brother plant some large planter boxes with veggies...
3. I didn't allow my girls to choose what to eat and what not to eat at the beginning of a meal. They were served one spoon of each item (about 3 bites), and were required to eat at least two bites of everything before they could request something else or leave the table. This made them keep trying foods they didn't think they liked. Some foods they learned to like, others they prefer not to eat now that they are grown.
True story: I used to HATE squash. It didn't matter how you cooked it, I hated it. But, as an adult I tried squash again and now I LOVE it. So, I'm not foolin' our tastebuds really do change. Share with him a list of foods he knows you eat. Then, share with him some of those that you had to acquire...
Also, If you can help him research all the vitamins that his body needs to be healthy, and what different foods do give us each of those nutrients, it might help.
Then, have him do some research on what is in those foods he does like, like Hot Dogs. When he reads the package he's going to wonder "What exactly is...by-product"... go Ask Jeeves... Then, ask him if he'd rather a hotdog or... Broccoli/Cheese/Rice casserole.
Good luck!
T. Cobbs
(Yeah, I'm a science teacher... this post probably sounded like a nerdy science teacher...)