Firstly; there is no way an 18 mo old should still be eating jarred baby food. He should be eating pretty much what everyone else is eating!! If you put food in front of him, he will eat it, just don't give him another option.
Anyway, I would check out a few of the things the others suggested. I found that morning snack had to be scaled back to basically a piece of fruit and a cup of milk. Much more than that, and the kids won't eat lunch, but are then starving halfway through naptime and waking up.
Like the others, I have found that simple is best. Stick with the basics and add new things one at a time. If I am introducing something "exotic" I will do it with some of the tried and true things as well. I always serve a couple things at a time, so that the kids can choose. IE: chicken, noodles and veg. If they don't want one of those, they can eat the others. However, if they want seconds, they must eat all of what was on the plate first. Except for the real little guys, and lets face it, those aren't the ones with the issues.
I will not make different meals, or cater to picky eaters. I don't mind leaving sauce off of noodles or not putting jam on someone's toast though. I agree with asking them to stay at the table for a certain length of time. And if someone is throwing food, or (past the age of about one) making a deliberate mess on the table/tray, the food is removed for a period of time. That usually means they are done, and getting bored anyway. IF they are still hungry, they are reminded that food does not go on the floor and given another chance.
I think it's about finding a happy medium between serving them the foods they want (chicken nuggets and fries everyday much??) and the very healthiest things we want them to eat all the time. I found that casseroles don't go over well, so I don't serve them. No sense in wasting food. I will separate the food groups if need be, so that the food gets eaten!