Fact of the matter is it is his choice whether to eat or not. Both mine did exactly this at the same age - it is part of asserting their independence & testing the power structure in your family! Also, thier food intake at this age slows a lot as that mad growing phase is over.
We just offered what we normally would for dinner, if it didn't get eaten, then the only other choice was fruit or nothing. They went to bed dinner-less MANY times & woke up the next morning only to eat a huge healthy breakfast. All bets for treats were off as a regular thing - though still occassionally for lovely behaviour or certain accomplishments.
I spoke to three Paediatricians about this and all said that:
1. kids WILL NOT let themselves starve
2. do not alter what you offer them to eat
3. no need to introduce suppliments where there is no health issue or big weight loss, they are all full of sugar and very addictive to an otherwise healthy child
4. don't let it become a personal issue for you
5. start to assess what they eat on a weekly rather than daily basis - you'll find they do quite well
6. one decent meal a day, surrounded by nutritious grazing is fine for a child this age - they'd rather play anyway!
Good luck.