We're going through the same thing. DD is not eating much at all, no matter what I make her. So now I am making her one meal (for each mealtime) and not giving her anything else. And some snacks in between. No more bickies or junkish things if she hasn't had healthy food. I give her about 20mins to eat at the table and always with me eating too (always at the table for meals, snacks it depends on what she's having) and if she's just playing silly buggars we take it away and she goes to play. I feed her every 2 1/2-3hrs so she's not going to starve. I'm not giving her milk in the day time either anymore to try to increase her apetite, though she has 2 150-180ml bottles at night. As Deidre says, try not to stress or make a big deal if he doesn't eat much, he won't starve and it'll just make him realise that he has power with food if you do stress and fight with him.
He will eat when he needs to. HTH!