As Lorna has said, there are folk on here that live with cockers that resource and / or food guard who will offer better advice than I can give.
Your boy is young - he's still a puppy - in my experience he won't fully mature behaviourly until he's the best part of 3 years old.
He will be in a very confused state at he moment. I work with a spaniel rescue - we always advise that it can take at least six weeks for a dog to settle into its new home, smells, level of noise/light, routines, commands, food - you name it, it's all new and probably quite overwhelming.
You have clearly thought through getting Tony and have taken the time to join COL which tells me you want the best for your boy. My initial thoughts are that the behaviourist is a good idea - so long as they don't use any form of aversive training - positive reward is good but please don't reward with treats. However much you want him to know when he's behaving, rewarding with food this early on may exacerbate the problem. Consider, you are hungry, in a strange environment and the lovely people who are being really kind to you (except when they lock you up for being a 'puppy'
- just kidding) taunt you with the best treat you've ever had in your life and then take it away! When they bring it back I'd likely try to get to it as fast as I can just in case it disappeared again
if it was wine they'd be no stopping me
I'm not trying to be hard or criticise - just wanted you to consider from Tony's point of view. I'm guessing that you don't know his history other than what the RSPCA have been told? He may have been taunted over food, punished by having his food taken away while he was eating or not fed at all. At 7 months old he's barely through the teething age - there are lots of threads on here about the dreaded cockerdile phase - I can imagine his previous owners probably despaired he would ever get through it. Of course, that's assuming he only had one set of owners...........
We've just rehomed a 5 year old cocker spaniel from a friend who has quite a few dogs. She's the most soft natured dog, well fed and cared for but will scavenge if any of ours drop even the tiniest morsel - she has no need to, we would give her more if she was thin or we knew was really hungry but because she's always eaten in a hurry with lots of other dogs she behaves this way. We are lucky in that we know everything about our newest girl.
My best advice would be for you to set a routine for him, so he knows when to expect his food, or a walk, or play time (he is still a puppy and will have lots of bursts of excitable energy) and reward with cuddles and soft stroking. Although he doesn't appear so he is probably just as frightened of you as you are of him right now. At his age I am certain that it's not deliberate but you may have a long journey to help him overcome his fears/guarding or whatever other diagnosis/label the behaviourist gives him.
In the meantime, we are all here to help you in that journey. There are few cocker spaniel owners who don't have some story or experience to share - they must be rare and probably not on here
I hope you and your arm are ok?
Welcome to COL - I'm Jayne and owned by Pearl (show type blue roan age 5.5) and Coral (age 18 months black and white working cocker spaniel) I also look after my OH cockers: Sally - golden working cocker spaniel age 2 and Purdey - black and white working cocker spaniel age 5.5 who belong to Mark