My ds can get like this at times (and we too have a younger to worry about). Things that have helped:
1. Making our environment more calming. We keep *very few* toys out at one time. The rest are boxed up in the garage and we just rotate them weekly. We keep the main room he plays in as clean as possible. The more stuff we have out, and the more toys there are to pick from, the more likely he is to get like this.
2. If he starts hitting, it usually means he is tired or needs to be bottlenursed (ds is adopted) so that he can regain his equilibrium. Often, if I put him down for a nap as soon as he starts getting tense, that curbs the violence.
3. Deep touch and heavy work. Some kids who are like this are "sensory seeking." That is definitely the case for ds. An occupational therapist would likely recommend deep touch and heavy work. Deep touch means that they need to be squeazed in bear hugs, have their joints compressed (my ds loves it when I start at his shoulders, go to his elbows and wrists and hips and knees and ankles, squeazing away as I go), have some weight on them (such as a weighted blanket), get wrapped up tightly (ds likes being a "burrito" in a blanket while mommy pretends to eat the burrito), or get squished in a "sandwich" between pillows. 10 minutes of deep touch play alone a couple times a day can make a world of difference. Last night we played like this for a half hour with ds before bed, and it eliminated his usual before-bed meltdown (that happens no matter when we set bedtime). Heavy work means things like pulling a wagon filled with heavy books, carrying a heavy jug of water from one parent to another and back again, pushing a cart with a brick in it, and so forth...all in the spirit of fun and play, of course. Heavy work and deep touch provide the child with proprioceptive input, which is calming for the sensory seeking child who needs this input neurologically to process information.
4. Things to aid with transitions are helpful. ds does not do so well with transitions, so it doesn't matter if we do three great activities in the day because the transitions between the activities just make life hard for him. So, rather than cutting the activity out, we do things to aid transitions. Visual signals that they are coming, talking him through them, and so forth...we are still working on this.
5. ds benefits from routines that he can manage independently. For example, his coat is hung on a low hook he can access by himself, and he is slowly learning to put it on by himself (Montessori method). His boots are low on a shelf in a closet he can open on his own. He has snacks and water that are accessible to him in the kitchen, on his level. He thrives the more order and structure there is...so we do things like teach him to clear his place after eating, and so forth. Order. Structure. Independence. OSI...three *key* ingrediants.
6. Limiting access to stuff he teds to destroy. For example, he has lots of board books low on a shelf (though he has destroyed a couple, he doesn't destory them nearly as easily as paper books), but most of the paper books are up high where he can't reach them. We get them down to read them to him. This helps him be successful, which creates some momentum of its own,
7. Concentrating activities...this means stuff that takes concentration. ds thrives if we give him an activity that requires focus, as otherwise he flits from thing to thing (lowest attention span known to man, I swear!). It has to be stuff that I know he can be sucessful with, but also stuff that has some challenge to it. For example, at IKEA we found this peg that you stack wooden rings onto that has some rubber grip to the pegs, so it takes a little more focus to slide the rings down the peg. When he was a little younger, he could spend 10 minutes with this! I am not thinking of a more current example off the top of my head, but hopefully you get the picture.
7. ds is a kid who gets easily frustrated. His frustration doesn't show in typical ways though. When he can't do something the way he wants to do it, he will go do something else...but *then* when doing the other thing, he is more likely to toss it around even when he isn't frustrated with that particular thing. Sooooo, we work hard to balance having appropriate challenges for ds (toys he still has to work to do what he wants to do with them) while also keeping frustration to more of a minimum than we might for another child.
8. With a younger one in the house, I have come to have zero tolerance for violence. We have dealt with it using a multi-pronged approach ranging from working with him (when he is not feeling the need to hit) to practice gentle touches (rubs, etc.), redirecting him to hit the couch when he wants to hit (he thinks it is hilarious, especially if I shout "bang" everytime he hits it), having him take a break (not time out...he gets up when he wants) by sitting on a chair when he hits...followed often by a nap or bottlenursing because usually he is tired or needs some help getting some equilibrium, being very firm when he hits, and so forth.