My little man can also be a fussy eater, and at times I worry he's not eating enough either! When he's like this, I give him 3 glasses of sustagen a day..to make sure he's getting some 'nutrition' of some description! Pentavite drops are meant to be good too, and I"ve been meaing to give him a kids multi vitamin.
At times, all he wants to eat is things like 2 minute noodles, he'd eat them until the cow comes home! So I buy those packets of little frozen vegies and add them to it, so he's getting some vegie content! He loves corn atm..so I"m reaping the benefits and making LOTS of little things with corn in it...corn and chicken soup, creamed corn on toast, corn fritters etc...I also grate up vegies and throw them in things so he won't notice! He refuses to eat fruit, so I have to make him fruit smoothies with yoghurt and honey..it's annoying, but at least he's getting it somehow!
I have also recently introduced a sticker chart to encourage him to eat more 'healthy' things. Even if he doesnt' eat ALL of it..I try and make him eat, say, 5 bites or something. If he does that, he'll get a sticker..if he gets 10 stickers, he gets a small treat like an ice cream, little car, book etc. This has really helped.
I think we all worry at some time whether our kids are eating enough, but as someone said, if they're really hungry then they WILL eat!