Eating

Eating has become a challenge over the last couple of weeks. Little man has been sick with a sore throat, and understandably he wasn't too keen on eating anything other than yoghurts and custards. He seems to be over it now, but his appetite hasn't returned to normal yet. He would always have quite a big breakfast, but the only thing he will have for breakfast now is a bottle of milk. He isn't really keen on lunch, just nibbles, but will have some dinner (depends what's on offer). He certainly hasn't lost any energy and according to our scales has put on weight. I'm hoping that if I just keep offering him food he will eventually go back to his normal eating. He is happy and energetic and drinks plenty of milk and water. How long should I let this go on before seeing a doctor?