First of all, I'm glad you've realized that communication breaks down and that consequences have no effect. Now that you know they don't work, you'll realize they're only harmful. Please refrain from punishment for a behavior that is out of the child's control at that moment (full disclosure: I'd say always refrain from punishment, but that's besides the point here).
Please note that being physically held during an upset can be terribly distressing. There is also a non-negligible medical risk involved, such as restraint asphyxiation. The safest thing to do when a child is acting out is usually to take the sibling to safety, instead of restraining the out of control child. I'm not talking about what's right or fair, I'm saying that only with regards to what prevents harm.
If you feel that you must hold your child, it is generally better to move with the motion of the child, and gently direct, say, their arm away from the intended target, as opposed to using excess force in the opposite direction to hold their arm steady, as this tends to increase arousal and escalate the situation. It is also better to hold repeatedly in several short bursts (seconds) than for an extended time period. But again, know that physical restraint is risky and distressing to the child, and seek out other options.
Above are some things you'll need to know when this happens again, but moving forward, you obviously want to avoid such situations.
After an incident, try to debrief the situation, and analyse triggers. It is helpful to ask yourself what you were demanding from the child in the moments leading up to the acting out. Demands can be things like expecting him to understand an overly complicated instruction, expecting him to socialise in a situation he isn't comfortable with, or expecting him to carry out a task which he normally has no problem with but now he's being asked when he's mentally fatigued. Obviously, also a whole array of other possibilities; I included some examples of demands that may not be obvious why they're taxing, to broaden your thinking about the topic.
If you can't find triggers or environmental factors that may have contributed, it may help to debrief with the child. Something that is sometimes useful is to draw pictures of your interaction, like a crude cartoon, from where you entered the room, where he was sitting, and who said what up until the problem behaviour. Ask him how he felt at the different stages, and what he was thinking. See if you can tease out some information about his feelings and thought processes that could be useful but he might not think to share, or even recall before you ask about that situation explicitly.
From your description, it seems like the onset has been rather sudden, which makes me think that there is an underlying stressor, such as something bothering the child, making him prone to lose control when faced with other challenges. You may or may not be able to get to the core of that now, or even find out if this is the case. Perhaps you'll need to adjust to the fact that he's more easily upset for now, and try to accommodate that. Perhaps there's a problem with a friend at school or something else that is troubling him that you may be able to address directly. Ask the child's teacher if they know of any conflict or if they have noticed anything that has changed matching the time of the onset of this change in behaviour. Ask the child.
We don't know what your child is going through. He may well need some extra attention and comfort in between these sessions as well.
Your question puts a lot of emphasis on what happens when he's out of control. This is almost never the right time to solve a problem. You cannot reach him at that point. Accept that he does not have access to the reasoning part of his brain at that point. I'm not qualified to comment on whether such frequent losses of control is common in 7 year olds, I don't even have one myself yet, but given that control is lost, there is nothing exceptional about the behaviour you describe.
The solutions that will eventually work for you are the ones that are being enacted before he loses control. In the chaos phase, you can only stay calm and give him what he needs to feel safe. And protect others from harm preferably by moving them out of harms way. Do not confront or otherwise escalate his distress.
