Your child is now beginning to open up to receiving new emotions and relationships. He is making friends in school and socialising with other people as well. There will invariably be events that might make your child happy or sad and he should now start learning how to control and channelize his mood so that it doesn’t hamper his behaviour. It is your responsibility to keep a check on the kid’s mood and help him come out of any bitter phase.
What you can do
The first step should be to observe the child. If the kid is not eating properly or getting angry on small issues then it is a sure thing that something is bothering him. Be a friend to the kid and ask him about his problem. Kids can be upset about bad events at school like failing in any activity or someone bullying him. Help the child to get over the despair and teach him to improve his abilities so as to tackle situations and people. Your time and support are what the child needs to manage his changing mood.