
One thing that I must say for now is do NOT tell him off.
He's already stressed and anxious; adding punishment of any kind will only pile on more stress.
You end up in a vicious circle: he is stressed/anxious about being around the others, so he has a go. You tell him off. Next time he's around them, he's stressed and anxious about being around them; and he's also anxious that he might be told off, so in his heightened stress, he has a go. You tell him off. Next time he's around them, he's expecting a telling off, and he's stressed about being around them, so he has a go....
So on and so forth. Anticipatory stress like this is horrid for the dog - it means they are on edge long before they are even in the situation that preceded the original telling off, and it makes them umpteen times MORE likely to do it again. It can also lead to your very presence being a source of stress, if the other dogs are around.
So, flip it. Instead of concentrating on what he IS doing, concentrate on what he DOESN'T do. So, every time he looks at one of the others and DOESN'T do anything, reward it (not necessarily with food as he's being guardy - a good fuss, a play, anything he finds rewarding). Build a marker word (Good! or Yes! - something he knows means a reward is coming) so that you can catch these moments quickly - they will happen quickly at present so you need to be, too. And don't just mark the odd good moment - mark every single good split second that you see. You need to start building positive associations with the other dogs (and with your presence around him and the other dogs). It can be very effective - I once had a mini schnauzer pup in for grooming that Remy absolutely detested, with 5 minutes, a clicker and sausage bits he went from trying to have a go at her to loving and playing with her!
Now of course, this won't be the whole solution - you do need a behaviourist in, he sounds extremely stressed and you need someone to observe things first-hand. But it will help.
I'd also maybe consider revisiting the IBS-type problem: look for anything that might be connected to that (e.g. vitamin deficiencies through poor digesition causing problems, or pain - I have IBS and it is excruciating when it flares, but in 25 years of it only two people have ever actually seen me suffering through a flare up and I am considerably less stoical than any dog!). There's even a possibility that the others were near him in a previous bout of pain and it was associated with them - the little dog I look after during the days started her dog aggression that way, with a tummy upset once. Definitely an advantage to seeing a veterinary behaviourist too in that respect, if that has played any part.
If your own vets do get back to you, do still check out whoever they put you on to - I have heard of rubbish behaviourists being referred to by vets (and vets themselves often have little behavioural knowledge or are very out of date so might noit recognise any problems), so best to be sure.