It sounds like he eats a variety of things, which is good. The veggies could be beefed up a bit, so you might try hiding them in other things. Look at the Deceptively Delicious and similar cookbooks. Kids also like to dip veggies in other things - so if you can make raw veggie sticks he can dip in dressing or hummus, that makes it kind of fun.
Does he have sensory issues? Some kids have a big thing with taste, others with texture. I think it takes something like 20 times of trying a food to make it a taste they'll stick with. My son had a lot of this and sounds very similar to your child - except he didn't eat as much as yours does.
I would say "no" to the Carnation Instant breakfast and that sort of thing, anything with preservatives or additives. The pediasure has SOME stuff he needs, but not nearly all of it. He may also have an absorption problem - that is, he's taking in the food, but not actually absorbing it. Cellular nutrition gets the nutrients into the cells within 20 minute with a high absorption level, upwards of 90% (compared to, say, vitamin pills which are absorbed 15-30%) - so it's a lot of bang for your buck. I work in the field and we've seen tremendous changes in kids, both in terms of building muscle (more important than weight) and in terms of brain development (really important). That also helps reduce the sensory issues. There are a lot of ways to creatively sneak this in too. We can give it to preemies through feeding tubes, to third world countries where kids are getting practically nothing the rest of the day, and to kids everywhere. The failure to thrive kids turn around in a month to 3 months without fail.
I think the healthy brain development and bone strength are more important than weight per se, but if he hasn't gained any weight at all in a few years, you might want to look into this and learn more about it. Absorption, epigenetics, and pediatric nutrition are all heavily researched and there are still a number of physicians who aren't really up to date because nutrition just isn't at the top of the list in medical schools.
My son really turned around with this and also had better study skills, increased immunity, and fewer injuries from sports and outdoor activities.