Hi Serin.
Contrary to what you feared, you'll generally get sounder, more experienced advice than you will on Chat. :)
Two people on there made sense - Bemybebe (who I believe has relevent professional experience) and notmeagain, who, as she says, also does. I'm an independent rescuer and work with all manner of dogs and all temperaments as a hands on volunteer as well.
notmeagain said this:
"As a professional who works in this field you must:-
Take the dog for a check with the vet to rule out any physical cause
Speak to the rescue centre(but do not take training advice from them unless they are trained behaviourists)
Contact a trained behaviourist either APDC or APDT for correct advice and assessment.
Many dogs that appear aggressive to the unknowledgeable can easily be given a behaviour training regime and all the old behaviour can be corrected. However you do need skilled and professional advice.
In the meantime keep the situation at home calm, do not do things which you know will cause your dog to react, keep him quiet ,exercise on lead and do not allow him to meet people at the door. You may need door gates to give him space to be separate.
You must act on this quickly though -the behaviour will not get better on its own."
The question is, what do you want to do, how committed would you be to putting in the work and the money to change his behaviour? Bluntly there's no point in making recommendations and giving advice if you feel you haven't the funds or personal resources to carry them out.
While you give that some thought I'd like to ask you to pm me the name of the rescue (can't say I'm best impressed with them atm!). I can at least then give you an idea of what, if anything, to ask of them, what to expect and so on, whether you approach them for advice, for practical support or whether you approach them to return pooch. I may even be able to negotiate/suggest further ideas.
If you make the decision that he needs to go to rescue please look at where very, very carefully. A dog which has "bounced" back from an adoptive home is harder to rehome a second time, a dog which has bitten harder still - many rescues will just kill him. Dogs Trust would probably either refuse to take him or use "health reasons" to do so - ie that his mental health was suffering. They claim to have a no kill policy but I can tell you as a rescuer that there's no way that they would be able to take in, rehabilitate and rehome the number of dogs which might come into them with similar stories to your own.
Some rescues will take him back and rehome without assessment and rehabilitation - you don't want that either.
It's possible that your "rescue" is not a rescue at all, but a council pound - usually boarding kennels and the like which also run a stray rehoming pound as a sideline. If he goes back there he'll have a 99% chance of being dead by midweek. Another reason why I'm asking you please to pm me the establishment's name, so I can advise further if rescue is the route you want to take.
As for contracts stating that he MUST be returned to where he came from, well, there is no legal precedent, no test case as no rescue has ever enforced such a contract to my knowledge. So, it's doubtful whether they'd stand in court - they're a good way of ensuring that decent rescues get dogs like yours back and can rehabilitate or offer lifelong sanctuary rather than have you take him straight to the vet to be killed, that's why decent no kill rescue uses them. If yours is not a no kill rescue, if it turns out to be flaky and unreliable/dubious, as the cat friendly claim suggests, I'd ignore any such contract and, if I wanted the dog in a rescue, would choose a more appropriate one for him.
My own opinion is that the rescue he came from should offer full support, call in a behaviourist and so on. As I said, whether they will or not is open to question depending on who they are.
Or, going back to the beginning, this IS something you can do, it's FAR from the worse case I've ever heard of and it's perfectly possible to overcome it. If I can, anyone can - I dealt with very similar (funnily that was a Jackie too) when the owner dumped him on me having given up on him. He was rehabilitated, trained and rehomed and now lives with kids, 4 other Jack Russells... and cats! :)