These dogs are very stubborn, but here's the solution:
You pick a nice type of dry food like Artemis or Wellness or some such and you put 50 gram in his foodbowl.
And then you stop worrying!
When it is empty you put another 50 gram in.
He will maybe not eat for 3 days, but he
WILL eat when he gets hungry. You KNOW what you are serving is healthy and tasty, so this is a battle of wills.
He
WILL eat when he gets hungry. My girls didn't eat for 3 days and then they did and I have had NO problems since that
Be strong my friend
Btw: be sure that he always has water!