DS is now headed into Year 6 after the summer in a different class and house so he can have a fresh start both socially, behaviourally. This of course also means a near blank slate as far as any reward he can show off by altering his uniform or appearance goes and he's been told by teacher he must remove any stickers he gets by the next day or they will be considered non-uniform and this has been so since the incident (and he's had one or two summarily removed in this manner until he got the message). The special jumpers have also been returned to the school as we still had some at home, he also got his first internal exclusion. He also lost internet privileges for deleting the voicemail. Displays of external rewards he kept in his bedroom where also removed a week or so later.
We are getting to the bottom of it given the violent incident which happened in school but he seems so much happier now the pressure is off. We are working with professionals on his external locus and desire to signal virtue to others and we have stopped making a big deal out of him being a 'good' kid after having taken advice. School is reviewing reward system it uses.