Hi, V.. I'm really lucky, my 2 yo son is a good eater. I have found, though, that he is a GREAT eater when he "helps" me cook dinner. The other night I made a pasta dish with bowtie pasta, frozen spinach, can of tomatoes and a can of garbanzo beans. He dumped all the stuff in the pan for me and then stirred it. Occasionally I would say "Taste it and see if it's good." I kept the heat very low while he was there and eventually sent him off so I could really cook it. When it came time to eat, he ate his whole plate!
Also, he loves his juice, too so I still cut it with a lot of water. He seems okay as long as it has the flavor of juice.
You could also try giving him a chewable vitamin...
All that aside, I've repeatedly heard that kids get what they need and not to worry as long as they are eating.
Good luck.
L.