I am so sorry to hear this.

Some may say the principal needs to be informed about the bullying as soon as possible. You and/or your son will want to make the best use of the principal's time. You may wish to prepare, by having your son write a complete description of the incident, as it occurred, in chronological order. Creating a numbered list of points can make this easier. For example (using a fictitious scenario):
1. I was sitting at lunch table #5, unpacking my lunch.
2. I looked up at the clock.
3. B reached over and took my cookies from my lunch.
4. Until that moment I did not know he was there.
5. Then he laughed and stared at me.

All information in this example is manufactured.

You may wish to e-mail the principal. You might include
- the description of the incident,
- a statement that your son knows he is supposed to contact the principal if any such incidents occur,
- unfortunately the principal was not available,
- thank the principal for having a plan in place,
- request a back-up plan of individuals whom your son could contact if this situation were to recur.