When my nephews refuse to eat it usually helps to get them involved in the preparation of the food. They can help with washing the vegetables, mixing, and passing plates when the food is being served. They might even enjoy it more if you let them do taste tests and ask for their opinions. Let everyone know he helped to prepare the food and compliment him before everyone. They will eat more if they feel proud of what they did. Just be sure to keep a close eye on them in the kitchen.
I also used to have New Food Fridays where we would prepare something we've never had before. They always looked forward to that one.
If all else fails or if he refuses to eat for a long period, a pediatrician should see him right away to find out if it is something serious.