Oh, dear...getting kids to eat is such a nightmare (at times), isn't it?
I find with my two, they eat better if they are sitting at the table with a plate in front of them. I used to do the 'follow them around, feeding them' thing, and they just don't eat very much that way - too much else to see/do. So I insist on proper 'mealtimes' at the table.
When we sit down and eat with them, that seems to help also...they see Mommy and Daddy eating the same things that are on their plate, and that seems to spur them on.
If they really aren't eating at a meal, I just leave it. You can't force them to eat. Then I try to make the between meal snack quite small, so they will be hungry when the next meal time rolls around (I might move the time up a bit). This usually works, and I get at least one good meal in them a day!
Whatever else you do - don't give in and give him junk. If you keep offering only healthy food, then eventually he will eat the healthy food. Neither of my girls have huge appetites, so they just don't get much junk at all...if all they are going to eat is this tiny amount of food, I make sure that that tiny amount is nutritious!
This is a lot easier said than done, but try not to worry - kids won't starve themselves. I think they must need much less than we think they need - at least mine seem to (sigh).
Hope someone else has some specifics for you about encouraging fruit and vegetable consumption!