The Explosive Child is not really about managing anger.
It is a good book and a useful approach but a lot of people find it frustrating as the title makes you think it will help you with what to do when they explode, which it doesn't really. Or that it will help children explode less. Which it can do if you are able to follow it 100% and drop close to all demands, but for most people they aren't at that point when they read the book.
What it is is a very good guide for radical acceptance, seeing the child's perspective, and communication around that, problem solving and figuring out out-of-the-box solutions for predictable, repeated areas where you and your child tend to clash - these are essential skills for ND families, IMO, and the messages from the author are extremely valuable, but I would expect that this wouldn't be hugely practical for you at this point in time. (It is a good book, so don't send it back!)
What I would recommend particularly if you want something that will go well with the WIC course is this online course - it's free if you choose the no-certificate version. You can do it at your own pace and it's useful from the first lesson. It's much more positive and it explains some other useful things about rewards, consequences, intrinsic motivation etc. It seems to be based on high quality evidence. There are also some really useful concepts in here such as - most people assume if threat of a consequence isn't helping a child change behaviour, you should increase the consequence. Actually, it's much more effective to move the goal closer. So build a stepping stone and aim for that stepping stone instead, once you're stable on that stone you can aim for your originally wanted behaviour. And if it's not working, instead of increasing the consequence or reward, reduce the gaps between each stepping stone. I don't love the fact that they immediately jump to compliance as an example of a behaviour to target, but you can use it without targeting compliance specifically.
https://www.coursera.org/learn/everyday-parenting
Most likely the school reward jar has stopped working because it is either not specific enough so he doesn't know what he's supposed to do to earn the reward, the "payout" isn't coming quickly enough for it to be motivating, the novelty value has worn off and fatigue for using the preferred behaviour has set in, or it's actually targeting a gap which is too wide - they need to make a stepping stone to aim at instead. Though the one thing the ABCs course misses IMO is that sometimes skills also need to be explicitly taught or the foundation for that skill needs strengthening. Their theory is that if you reinforce lots of small instances of (or towards) the wanted behaviour, this encourages the child to repeat it and through this repetition they will practice the skill/behaviour and it will get easier for them. That is probably true if the unwanted behaviour is simply a bad habit that they have got stuck in. If instead, it's a strategy they are falling back on because they don't have the skills or ability to use a different strategy, then you might not get very far by trying to reward when they accidentally do the thing you want. Worst case scenario, you can even teach a child to mask their own needs by performing the "correct" response which is not actually working for them. (I apologise but I can't think of a useful human example, but an example in dog training would be discouraging a dog from growling, which then means that when the dog is stressed it has no way to communicate this and while it may appear less aggressive in the short term, it may mean that the dog is more likely to bite with no apparent warning.)
OTOH though since a reward jar is designed to strengthen and reinforce practice of a behaviour that you are trying to develop, it doesn't matter that items don't get taken out of it. That would be like saying every time you get a maths question wrong you're undoing a bit of maths you've learnt. It doesn't work like that. It's not supposed to be hard and unforgiving of mistakes. It's supposed to be supportive and encouraging of doing the right thing.
(I also think it's fine if you previously used a strategy where taking away points or tokens previously awarded worked - it doesn't really help to have such blanket statements about what never works or always works because all children are different.)
I agree it does not help to add consequences at home for behaviour in school. Support the school's consequences yes but no to adding extra ones. One of the things that can really help as well if you have a child who struggles with emotional regulation is having a framework where the structure of rules and consequences are essentially framed as external to the child-parent or child-teacher relationship. If the child sees it as "She just hates me and is always getting me in trouble!" that doesn't help motivate them to behave at all - it just causes the relationship to break down. When you can be like "Oh no, you've lost a point now because you did X and that's against the rules, remember?" it's much easier to position yourself as on your child's side/how can I help you so you don't lose another point today, let's make a plan to keep your points tomorrow. But that means you have to stick quite rigidly to the system, don't manipulate it in order to benefit you (e.g. letting something go because you think they tried really hard or you feel guilty or they had a bad day so it was excusable, increasing a consequence because a behaviour was extra annoying or threatening to increase it to win a power struggle etc.)
Children who don't usually have difficulties managing their behaviour but occasionally make mistakes as everyone does can handle the occasional bit of parental disapproval/disappointment, but if you have a child for whom behaving well is a constant challenge, it REALLY helps if they can see adults as united with them against the very tedious-but-sensible-really system of rules, boundaries and consequences. If they get into a position where they feel it is them vs the adults, then it tends to make things very difficult indeed.