Luckylady74,
Do you speak to the head often?
I know it varies from school to school but in most primaries you would have been sat at the back, meaning those immediately in front of you would have been year 6 students.
I would speak to the head and explain how wonderful it was to visit the school to watch the assembly and especially to see your ds get recognised. While you understand that your ds latest 'thing' can be a little embarrising/funny you were extremely dissapointed that the older children found it necessary to enliven their dull lives by publicly mocking your son in such an open and disrespectful fashion. Nor had you expected it from what you have come to see as a warm and welcoming community.
Keep your tone friendly and leave it there, unless they ask for more information. In which case just answer what they want to know. Believe me it will get much better results then if you went in ranting.
Most HTs are concious of their public image, and often invite governors and possibly local councillors/mp to these events to show how wonderful they are. Plus they are aware of the old truth, if you have a good experience you are likely to tell one person, if it is bad you are likely to tell ten.
You may or may not get feedback, but I would be surprised if their boring existance is not interrupted with a 'chat' from the HT.
I pulled a similar trick over something I wasn't happy about in one of my daughters schools. The teacher was removed from the class immediately, the riot act was read to the class and all the boys had a note to take to their parents to come in and speak to the HT - it was a little more serious then what you have described.