2 of my 4 are picky eaters- but in different ways. My oldest is only picky at school. He's 12 now- and it's really not better. He is bothered by the smells of the cafeteria and/ or the food of who ever is sitting near him. For some reason, there is very little he will eat @ school, but @ home, he eats great. I spent much of K, 1st, 2nd, 3rd, 4th ,and 5th grade banging my head in the wall. I seriously tried EVERYTHING- even taking him to a nutritionist and having the doctor "talk" to him. If I pack things he doesn't like, he simply won't eat- at ALL- so now after years of dealing with this, I give him what he will eat, which is a bag of Sunchips and freeze dried organic pears. I know, I get the "worst mom" award but he wore me down. When he comes home from school, he has a turkey sandwich, AND some sort of snack, like bananas or fresh pear slices w/ cinnamon sugar sprinkled on it, or slices of cheddar cheese with crackers. After all that, he still eats a big dinner- sometimes less than 2 hours later.
Anyway, my 8 yo is picky, but slowly showing a little improvement. He decided out of the blue about 6 months ago, that he wanted to try a hamburger- and loved it. What I found with him, which I suspect might be the case w/ your son, is that he is bothered more by textures of foods and the way something looks than flavor. For example, he loves freeze dried strawberries, but no amount of coaxing can get him to eat a fresh strawberry- he doesn't like the texture- it feels "slimy" to him. My husband has tried the "you can't leave the table until you try_____" He eventually tries it, but you'd think we were trying to get him to taste Drain-o or something. The more pressure that there is on him, the worse he does. I found that when we back off a little, he will eventually decide he wants to try something.
I think it may help some that his 6 yo brother is a little "foodie"- with him, it's easier to list the things he doesn't like than the things he does. Doesn't like any kind of breakfast cereal, tomato soup, or grilled cheese sandwiches, but he loves brattwurst w/ horseradish mustard, for example. He loves watching the Food Network with my husband and loves going ouside with my him when he's grilling. He's also very vocal at the table with his "Mmmmm's" and you can see how much he enjoys his food- that seems to pique the curiosity of his 8 yo.brother. enough to want to try something sometimes.
Anyway, after struggling for years w/ picky eaters, I finally came to the conclusion that they'll survive. Some kids do really well with a "this is what's for dinner- take it or leave it approach and some don't (yes, tried that). I had to stop worrying what people must think because they don't live with MY child and have no idea.
So, to answer your question, if it were me, I'd let him eat what he likes. When he does try something new, but doesn't like it, tell him that you are so proud of him that he at least tried it. Try not to make too much of his not wanting to eat something, but praise his sibs when they eat well or try a new food. I'm thinking that if you try to set up a reward system, his sibs will feel like he's getting special treatment.
Anyway, HIH !!
Good luck!! :)
ADDED: Amen, Carrie T!