Do you make a big deal about him eating his veg? It's not unusual for them to find these little pressure points to exert a little autonomy over somethign they know you care about.
It sounds like he eats really well on the whole, so try not to worry too much. Just continue to offer. Rissoles are great for hiding vegies if he likes meat. Trying different shapes, textures and cooking methods can also help - like Yeddi says sometimes they won't eat it one way, but are happy to have it another.