I really echo what Baileys has said.
ds (aged 10) kicks off a lot, and we go through cycles, he is great for a few months and then we have a bad patch.
He is like Jekyll and Hyde when hungry, and we have spent years trying to teach him to recognise this in himself and learn that he needs to control himself and eat. He is not there yet, but better.
Once he kicks off, he very quickly gets to the place where he can no longer negotiate, or control himself enough to respond to sanctions, so it is no use saying ''if you continue xx will happen'' as he is past caring, so we have learned that once he is kicking off, what he needs is to go to his room and calm down, and then we can discuss it. There is one consequence for the whole episode. Otherwise we ended up with removing one thing, then another, then another, all during one temper tantrum.
We have talked about it many times when he is calm and he understands that going to his room is for him to calm down, and he needs to come down when he is ready to talk, that puts the responsibility back on him. If he comes down and starts being mouthy again, he goes back until he is ready.
We have seen huge improvement in the last year or so, but he still reverts at times, he was dreadful after school in sept until he had settled with his new teacher, and I expect him to be awful next month, after he comes back from he residential school trip, he will have a great time, be well behaved and then melt down all over us when he gets home.
We have also deliberately made times when we show him love. He has half an hour mummy time on friday nights when younger dds are in bed, and it started with us watching doctor who (not suitable for younger ones, so we recorded it) We gave him a few things that were recognising he needed responsibility, and he loved those, we also let him earn some extra pocket money, and let him negotiate a different bedtime routine, all about recognising he is getting older and his voice is valued in the house.
Last point (sorry about essay) he is HORRIBLE to dd1, who is 2 years younger. It is an old, ongoing problem (he has no probs with dd2) We have sat down with him, and acknowledged that younger siblings can be a pain, that he doesn't want her there, and that he doesn't like her. It is important I think for them to realise that these are normal feelings. On the other hand, we love her, and she has the right to live in her home and be treated with respect, just as he does, so the rule is respect each other which means no hitting/kicking/nasty teasing/winding up etc, if you can't respect, remove yourself from the family space, ie go to your room.
I think the fact that your ds has improved and is now slipping back, shows that the techniques you used worked, and he is now having a blip, reinforce what you did before, go back to basics again, and try and work out if something has triggered it, but what you did before worked and will work again.