Sorry that you and your boy are having to deal with this! Fear issues are tough.
As a CVT with over a decade of experience, and also as an owner of an extremely fearful and touch-sensitive dog, I've experienced these issues firsthand many times, both as an owner and as a veterinary professional. This is what I've learned.
First off, Magwart and LeoRose have both provided some really good resources. I'd definitely take a look at the links they provided. In addition, take a look at Laura Monaco Torelli's YouTube videos about training dogs to voluntarily allow veterinary procedures. The chin rest behavior that she demonstrates is very good for situations that you might run into at a vet clinic.
When choosing a vet, be sure to work with one implementing the fear-free protocols that Magwart linked to. Veterinarians who are familiar with these protocols will be able to better assist you in working with your dog. If your dog's behavior is as extreme as you are describing, your vet most likely won't be able to handle your dog right away, but should offer support and be willing to work with you to achieve that goal.
If there are procedures that need to be done in a timely manner (ie, the rabies vaccine) consider enlisting the services of a mobile vet, just in case it's the setting that is the issue.
The best approach with a fearful dog, especially one as panicked as yours, is slow, systematic desensitization coupled with teaching coping behaviors. Please don't punish his fear displays. This will only make things worse.
Purchase a basket muzzle, and work on getting him used to wearing it at home and while out and about before trying it at the vet. Start with short, low stress outings to the vet. Drive to the vet and sit in the parking lot, in the vehicle for a few minutes, feeding treats. Then progress to walking around outside, again, with high value food rewards. Then walk into the lobby, and walk back out before trying to interact. Gradually progress to using the scale, sitting in the waiting room, etc.
Utilizing a veterinary behaviorist is an excellent idea. The key word here is veterinary. Lots of trainers throw around the term behaviorist without a full understanding of what such a title entails. There are two instances where the title of behaviorist is appropriate. The first is an applied animal behaviorist, which involves years of study and a PhD. The second is a Veterinary Behaviorist, which is a veterinarian who specializes in behavior. For the extreme fear that you describe, I would contact a veterinary behaviorist.
I've enlisted the help of a veterinary behaviorist when dealing with my fearful girl, and the information and help given was invaluable. I can't recommend it enough. In addition to providing insight as to what is going on and why, veterinary behaviorists can also prescribe medications that can help during the behavior modification process. With my fearful dog, we started with a calming supplement during training, and weaned off of it after a few months of work. We also had plans in place for the use stronger medications if need be, but it wasn't neccessary in our case.
Hope this helps a little, and let me know if anything I mentioned needs clarification or if you have other questions