If this has come on as he gets to his young adult /pushy adolescent phase, which seems to be the case from your post, I would think it is much more likely to be about him testing boundaries and seeing how he can influence people than about his having suddenly and inexplicably become fearful of people, particularly if he is targeting "easy" (i.e less confident) victims .
In some cases dogs do not become more fearful on the lead but rather they can gain confidence from it, and from pulling thier owner into the situation for back up, especially if the owner does not have the dog's respect.
Lack of respect for the owner which can be a reflection of lack of confidence in the owner to manage situations can also encourage the dog to be overly possessive or overly protective of that owner which is a situation that can also be heightened by a lead as it has the dog in closer proximity to what it is guarding, i.e. it's owner.
After all GSDs are bred to be confident, assertive, territorial, security dogs and their work abilty relys on their handler being able to control that potential aggression, otherwise they can make their own decisions as to how, when and where to use it.
Also a lead can give them a clearly definated area to claim.
It could be a wise precaution short term to muzzle train your dog while you are looking at a longer term solution, although muzzles are not infallible as dogs can remove them, hence the training so he is happy to wear it, and you still need to have him under close control as they can still do damage with claws and weight, and still scare people, but it may help you to relax a little more and it so long as it is used properly it greatly reduces the possibility of him biting .
Obviously I know nothing about who you are planning to see but please remember anyone can term themselves a behaviourist and dealing with an large, aggressive GSD, from whatever motivation, is a very different proposition to running a puppy class so ask questions before commiting to seeing this person about their experience, qualifications, insurance and in general terms how they would propose going about helping you.
To some degree what methods would be appropriate would depend on what the dog's motivation for the behaviour is and to some degree on the dog's personality.
It would also be a positive sign if they asked you a bit about the dog and his behaviour before any appointment so they know what they are taking on, and if they can help. I wouldn't be impressed if they asked nothing but were more interested in quoting you prices.
Having a very wide gene pool GSDs do vary enormously in temperament and some are relatively easily managed and trained and friendly with everyone, and don't necessarily all make good guard dogs as they might not be suspicious or aggressive enough.
Others can have a much greater predisposition to their security dog, working background and these are can be much more challenging, or sometimes too much, for non-professional handlers to manage effectively.
Some GDSs can be difficult to manage because they are very nervous and reactive, others at the opposite end of the scale can be very difficult to mange as they are over confident and look to aggressively take charge in various situations.
Conversely the majority between these two extremes can be wonderful dogs.
. Sometimes just writing it down helps put into perspective.
. If it's worked before for them & the person/dog or whatever goes away, then they learn to react like this in future encounters.
he went through loads of questions etc with me and then we went out. He was really helpful and to cut a long story short, I am now a lot happier. Dizzy is a changed dog already, it's a work in progress obviously but the difference is amazing. Apparently he is very insecure when out of his home, and his barking was fear and warning rather than aggression. I was also told off for spoiling him 