Maybe you should give him a real breakfast and tell him to go and share it with the other people who live in his imaginery house
In order to get Elle eating breakfast again I set our dining area up as a Cafe as Elle loves cafes. I set the table up with candles and flowers and used her blackboard to create a menu. Each morning she ticks what she wants on the menu (I drew pics of foods) and then that is what she gets and is expected to eat. It has worked a treat! She loved Cafe Gabrielle. The cafe is now gone and breakfast is still being eaten.