Gosh, children are tough - I feel your pain!
Firstly, prevention is better than cure! Think about possible triggers:
Is he sleeping enough? He should be going to bed at the same time as if it were school tomorrow, with the same calm routine so he falls asleep quickly.
Anything changed in his diet? Is he having more ice lollies, squash or sugary treats than normal? Food has a MASSIVE impact on children's behaviour. Try to keep ultra-processed stuff to a minimum and cut down on sugar too. Stick to the usual mealtimes he'd have on a school day without endless snacking.
What's he watching? If he's having more screen time than he would on an average school day cut it right back. Parents seem to think kids need a screen for 'down time'. Audiobooks, colouring, jigsaw puzzles, threading beads, playdoh, snuggling with you while you read aloud, sticker books etc, all much better occupations for sitting quietly and recharging. Also, pay attention to what he is watching, as overstimulating programs could well be having an impact.
Is there anything unsettling going on in the background? You aren't pregnant? Sick relative? Best friend moved away? Marital strife? Death of a pet? Sometimes a tantrum over ' a drink' is just an excuse to let all those bottled up confused emotions out.
Is he underestimated, or overstimulated? Are you having trips out every day and planning endless Pinterest-worthy structured activities? If so take a few days chilling at home and doing nothing. Let him get bored.
Try 'love-bombing' him, if you have time, divide and conquer with your partner so you spend 20 minutes with the boys individually, taking it in turns each day, just playing with them one-on-one, completely undistracted by the other sibling. No phones or cups of tea, or half-heartedly playing while you attempt to see to chores at the same time. Just purely focus on the child you're with and what they'd like to do during that time with you.
To try and pre-empt a tantrum, do your best to avoid saying 'no' outright, and try and give him an option to control something. Let's say you get a 'Mum, I want some juice!' Instead of turning right round with a 'No, I told you no more today!' try an enthusiastic 'That cloudy apple juice we bought tastes sooo good doesn't it? We're saving the rest for tomorrow, but if you're thirsty how about some ice and a slice of lemon in your water? Would you like to cut it yourself?' It's maybe 50:50 whether this works but always worth a try.
Let's say it doesn't work on this occasion and he's getting worked up, 'I WANT JUICE NOW! IT'S NOT FAIR!' you can have a last ditch attempt to head him off by recognising the 'trigger' (as he sees it at least) and empathising with how he feels. 'I can tell, you REALLY want the juice! It's horrible having to wait isn't it?' then reiterate your 'no', but again, trying to avoid the word, and giving a valid reason. Rather than use the word 'but' which comes across as counteracting the sympathetic response, try 'the problem is' as that places you more on his side. 'I can tell, you REALLY want the juice! It's horrible having to wait isn't it? The problem is, the rest of us will have ours tomorrow, and it would be so sad if there wasn't enough for you to have some with us because you drank all yours today!'
Don't be afraid to talk passionately. I think we instinctively try to calm children by responding to their heightened emotion in 'it's not actually a big deal' tones, but personally if I'm having a rant about something I don't want the person I'm stressing to telling me soothingly to 'calm down', I want them to exclaim 'Oh my God, yes, you're right, that's crap!' even if I'm blowing whatever I'm moaning about way out of proportion. At no time ever in the history of the world has anyone felt calmer from being told to 'calm down'!
Let's say he's too far gone and he's really kicking off now. Remove yourself from his space but stay where he can see you/reach you. If you have time, sitting down, not looking at him or talking to him, with something to fidget with, shows that you're there to support him when he's ready, gives you something to help you regulate yourself (because we can't pretend their meltdowns aren't really bad for our own cortisol levels!) and the distraction of what you've got could help to bring him out of it. A couple of suggestions might be a tub of silly putty, a fidget spinner, a ball of wool, one of those 'rainmaker' things that's actually drops of oil, a squishy, a cheapo pad and black pen to 'scribble feelings' on, a poppet, mini bubbles (the favours you get on wedding tables are ideal), that sort of thing. Keep them in a box out of his sight/reach and just get one random one out for these moments. If you feel it's likely he'll go into meltdown mode out of the house or upstairs maybe slip one into your pocket beforehand. Anyway, sit there and fidget with it until he's calmed down enough for you to say 'Would you like a cuddle?' and he'll then either come for a cuddle or shout 'NO!' but hopefully be calm enough to hear you say, 'That's fine, I'm here as soon as you need one', And just sit it out as long as it takes. His curiosity on whatever fidget toy you have should help to bring him to you sooner rather than later. When he does, let him use it, and when he's calmer offer a cuddle again. Don't broach The Trigger until he's cooled down enough to not have the tantrum be reignited, He might open up to you when you're having a cuddle afterwards, but if not, try saying something along the lines of 'It's dreadful feeling so angry and upset isn't it? Why do you think you were feeling like that today?' and see if he opens up about a bigger reason that the
innocuous 'wanting juice'. If he insists that's why then again reiterate what you told him already, and offer him the alternative again. Then move on with your day.
I know it's easier said than done when you have a younger child to care for too, and quite possibly a job to do and a house to run, but it's laying some healthy foundations for the future. He'll learn that you understand him and care about his feelings, that you're consistent, that you mean what you say and can't be manipulated, that you're there to support him when he's lost control of himself and that you can offer him alternatives where he has some say-so.
Good luck, and remember, there is light at the end of the tunnel!!