I do think he needs some snack before bed, as we eat at 5:30 and he is hungry again before bed. I dont think a small snack of 4~6 crackers is a big problem, I just dont know how to mak it so he isnt overeating.
I would switch snacks to something he likes but isn't so obsessed with. Something with some more fat and protein in it maybe, that satisfies for longer. Save leftovers from dinner to reheat if he's still hungry. If you still want to allow the crackers, only keep a little in the house then when they're gone they're gone.
I have had this happen when we're traveling and eating different foods than what I normally have at home. If it is something I don't mind my kids eating I will tell them they can have it once a week and they can pick the day. Or, if it is something that I do not want them to have, I explain that I just don't have that item at home. Another thing you could do is make a healthier version of the food(s) you don't want him to have and have him help you. You could even get him involved in helping you choose the recipe.
I would consider dinner and snack two separate things. Just like lunch isn't dependent on breakfast, dinner and snack have nothing to do with each other. At dinner time, serve dinner. He can eat what he likes. At snack time, let him have the crackers and jello. If he asks for crackers and jello during dinner, just remind him this is dinner and that's snack. Snack comes before bed. If he's hungry now, he can eat whatever is on his plate (some, all, none, whatever) but snack isn't until later.
I don't think it's a good idea to set up food as a battle ground, or to make kids force down one thing to "earn" another. Everything I've read about kids and food says that parents decide what and when, kids decide if and how much. Dinner is ONE meal in a day. I'm guessing he's not eating junk all day long, so it's not a huge deal if he doesn't eat a perfectly well rounded dinner and has a few crackers and jello at bedtime. That's perfectly fine. I'm guessing it won't take too long for crackers and jello to lose their allure when they're always dependably available.