Thanks trish and FL!
I've started the process to see someone. I talked to the local child health advice group and will have an appointment with the local child doctor. We'll take it from there, see what she thinks and if she refers us to a child psychologist.
It's tricky. There have been so many little things with DS where we've wondered if it was something we should follow up more or not. Up till now we've generally been reassured that it's within the bounds of normal and decided not to seek outside help. With this though I think we've reached the limit of what we can do for him and hopefully seeing someone will really help with this and even other issues as well.
Hope all is going well with you