9 yr old DS - suddenly very emotional and angry?!

[Itsallgoingtobesodifferent]

I'm struggling with my DS (9) who was, until about 6 months ago, a happy sociable confident boy! Quite attuned to others, and generally cheerful. Now... every 5 mins he is having an emotional outburst....tears, anger, door slamming, then next minute totally changed and it's all 'I love you's, he also seems incredibly clingy? We spoke to him and asked teachers and it all came out that there were some issues with friends at school..but he says thats all sorted.....but emotions and outbursts continue...anything sets it off but mainly any kind of 'no'....i try and ask whats up but he always says nothing. Im not sure if this is ongoing social stuff he wont tell us about anymore, or the start of hormones???
His sister is 11 and has diagnosis of ASD (very academic, private diagnosis in year 5). we are aware there has been some focus on her last few months. I'm worried he is sad about it but don't know how to approach it with him...
Sorry for long thread. Any ideas/experience welcome....!