My 6 year old daughter is picky too. She will at least take a bite of something she hasn't tried before, but sticks to PB&J's, "chicken & fries", rolls, mac & cheese, etc.
Keep encouraging "new foods" - particularly things like pears and grapes if he's constipated. Our daughters will NOT eat veggies if they can help it, but will eat spaghetti with meat sauce. My mother said she used to puree up some carrots and sneak that in the sauce sometimes.
There are places we'll go out to eat that she would prefer not to go to, but we'll wait long enough until she's nearly hungry when we leave so that when we get there, she's hungry.
We just found it isn't worth the battle really, if you stick with things that are good for them. We really don't keep candy around the house...my oldest daughter's (and my youngest is getting into it too) favorite snack is cheddar cheese...I have a brick of it that I'll slice up.
They both love yogurt. My oldest went through a phase where she'd only eat "chef boyardee" stuff or spaghettio's. Now she wants her sandwiches without the crust.
You could try some of the juice boxes that have some fruit and veggies in them, or try putting cheese on the broccoli - who doesn't like cheese?! :) When my mom would make green beans, she'd cook them in a pressure cooker with some diced up onions and a piece of bacon. Maybe not the best way, but it might be worth a try.
Make sure that whatever you're wanting him to "snack on" - whether it's fruit/vegies or whatever - that it's something you're snacking on too. One thing that always drove me crazy growing up was that my mother would always say to have some carrots or something, but I never once saw her snacking on those things - it was always pretzels and pepsi. I guess I'm saying "lead by example". I'm sure you already are...that was just something that came to mind.
The alternative is to just keep trying new ways to make chicken...or to just keep feeding him things he likes until he gets sick of them and wants something else.
OH - one other thing we tried. My husband and I like to go to this pizza place in Geist. My daughter used to like it, then started grumbling she didn't want to go until we realized one night they had chicken nuggets. We told her she could get chicken - she was disappointed she couldn't get fries, but chips were okay. AND we get "DIP" - ranch dip for her chicken. Within the last 5 months, she's even begun asking for a piece or two of pizza again (along with her chicken!).
It's frustrating, but something we just decided not to pick too huge a battle over. She does eat yogurt, cheese, apples, bananas, etc.
Yogurt would help keep healthy enzymes in his stomach and should help with the gas too. Mylicon can help with any gas issues too.
When we go out, I'll try to make sure to order "something different" and my 6 year old has now gotten to the point where she'll ask for a bite of what I'm having sometimes too.
Just a few thoughts, for what it's worth...and good luck!