I neglested to answer your question about food. For the most part. diet does not have an enduring impact on focus or attention. Yes, it makes sense to avoid processed sugar in foods and foods that are high in fat content. And using Omega-3 Fatty Acid as a supplement (or in diet via servings of fish), along with foods that are rich in protein, is sensible. But overall, dietary adjustments will not be very useful as long as he is eating a healthy, balanced diet.
Now that the teacher is aware of this, it is up to her to be on top of it and monitor him. Your 'work before play' plan is excellent - such a plan almost always helps. Perhaps the teacher will have to do a similar thing by holding recess over his head if he avoids his work.