I would say he's stressed - my dd cries a lot and it's usually when she's stressed about something else. It's her body's way of coping with it. He's probably feeling anxious and nervous about starting secondary school. You need to talk to him when he's calm and agree a set of rules and sanctions. Once you get to the bottom of what's making him unhappy you can go from there. Good luck
Your story is VERY familiar to our family, its always special treats and occassions most at risk here too!
Don't have time to go into details but my DS1 has always been a tantrummer (and generally an anxious, glass half empty person). He was at his absolute worst during his last year at primary but had little personal insight into why he did it, we even got referred for family therapy (where they told us we weren't doing anything wrong!).
Miraculously he has improved an awful lot since starting high school, he is now in second year and is controlling his temper much better, I think peer pressure has a lot to do with it, it is not "cool" to be highly strung and he is much more motivated to fit in with his peers than his siblings. He still explodes but usually says sorry and i am able to deal with him more calmly as I realise that his bark is worse than his bite! He is like a reverse 'Kevin'.
Interestingly his brother has mild Asperger's syndrome and while DS1 doesn't I think he does have a sprinkling of AS traits, including a problem seeing things from others points of view (which doesn't come naturally to him but which he is slowly learning by experience!).
It will get better, to coin a cliche, keep calm and carry on.
DS (10.5) very similar at the moment - unhappy, belligerent, flying off the handle at reasonable requests, shouting in our faces - then charging round the garden with the younger ones, whooping it up in a fantastical game that lasted for hours, and sweet-as-pie for night time story and snuggle...what is going on with them?! Is it just 'their age'? I am trying really hard (after a total meltdown last week) to not rise to the bait, and also to pick my battles and not insist on the last word. Its hard trying to be the 'adult' one...