It sounds like you are trying your best and have routines that work for your child. If your child is “misbehaving", he is trying to tell the adults in his life that something is not right - the difficulty is knowing what.
What to do? Make an appointment to meet with the Senco and class teacher as early as possible next term to look at his academic progress as well as the behaviour.
It sounds most likely that there is an underlying special educational need of some type. ADHD has already been mentioned and his needing less sleep might fit with that. The Senco will tell you if the school is thinking along those lines, although they can not diagnose.
It may be that he is beginning to struggle academically. What does he say about how he is getting on? It's often easier for a child to misbehave to avoid doing a task that they think they would fail at. Difficulties are not always obvious from the start of school or school may have taken a “wait and watch” approach, or not tackled any learning difficulties as the behaviour issues have been more pressing or masked an underlying problem.
Again, the Senco and class teacher should be able to tell you what his progress is like. Some difficulties like Dyslexia can not be tested for until they child reaches about age 7, so ask if they think that might be an issue. However, before looking for any learning difficulty, always get eyesight and hearing checked in case there is a problem that has not been picked up, but is enough to cause him to struggle in school.
You can expect that School should be setting achievable academic and behaviour targets for him to work towards and putting in place strategies to help him achieve this. Also that they will action any referrals that might help (although in some areas the quickest way to get the help a child needs through Health is via the GP, but the Senco will tell you if that is the case).
One last thing to consider/ eliminate is whether his sibling's special needs are impacting on your DS?
Finally, to address your initial point, the teacher shouldn't have told the parent to speak to you directly about your child's behaviour in school. However, if the parent went on to also complain about something outside her remit, e.g. about something that happened out of school, the teacher might have directed the parent to speak to you.
If the parent approaches you inappropriately, direct them back to the school.