That's great about taking your son to see a behavioural specialist. Sometimes we have to outsource!
Understanding why your son gets angry will help you to help him deal with his reactions. Anger is normally a secondary emotion and comes after a different emotion. For instance, a mum who loses her child in the supermarket gets angry with him when she finds him and says" You're so naughty for running away like that! No treats for you today." when her first emotion on finding him was relief that he's ok. This is the message that she needs to send first.
If we can get to the first emotion, recognise it and deal with that successfully, we can often stop the anger building up. So helping your son to recognise and verbalise his disappointment, embarrassment, fear, frustration, or whatever that first emotion is, will hopefully help him to think before he gets angry. This can act as a cicuit-breaker. Role-modelling and teaching this at home would be a positive step.
All the best; Janet.