9 year old singing in the playground at school

My son is 9 and attends mainstream primary school. He copes very well and generally enjoys school. He does at times get overwhelmed and has sensory issues.  He sings lots. While this is lovely and he is very good at singing, it has created problems.  We have 2 other children and his singing throughout the night has kept them awake during the night and wakes them up early in the morning.  

At school he sings at breaktime while wandering around by himself.  While this is harmless enough, it started to create unwanted attention from other children, who call him annoying or have mimicked him. I have had several meetings with the school, but they haven't done anything about it, despite saying that the person on playground duty would intervene and distract him, or buddy him up with others.  There is a classroom they can access at break time but he doesn't want to go in there. He can't tell me why he does it, as he can't communicate his feelings.

I would like to get to understand why he is doing this, so we can support him if he needs it.  Ultimately, our worry is that when he progresses to secondary school, he will become the target of bullies and ridicule at breaktime. It is a constant worry as he is vulnerable and probably wouldn't recognise it as bullying either.  

I just wondered if anyone has any similar experience with their children? how can I find out why he is doing it? did you manage to stop the behaviour? how did you manage the behaviour or what did your school do to help?

Thank you - any help would be appreciated.