Jack is Afraid of Heights??

You'll have to set up situations to desensitize him.  It's likely the feeling of "thin air" all around him (where on a sofa it's enclosed at the back -- get my difference?)

You might start with several concrete blocks clustered and essentially use it like a 'target' like you would in conformation.   He walks along with you, steps up on the blocks  and treat.  Sit ... then treat.  Stay ... in other words build a small 'routine' around standing on there.  Then even add another layer or turn the blocks up on end and add some flat surface like plywood ... same thing walk up, jump up on and do 2-3 things. 

 Move it around -- dogs are VERY situational -- what they'll do at training they won't do at home and vs. ... then go inside -- same thing.  Inconvenient to haul around concrete blocks  but he'll get so this is no big deal and it will be STABLE and a good background.

Then get a board that's about a foot wide -- put a couple of concrete blocks in place and make a low seat/table-like thing.

Ok noo now you have wood and it gives a bit.  So go from the concrete platform or square plywood to this.  And either get more blocks or bricks so you can gradually build it higher

Build on this -- get creative and find things to put under the wood other than concrete.  THEN vary the material under him.  Thinner plywood (so it gives more).  Cover it with fabric. And then find a piece of metal -- you should be able to get something at a junkyard -- a piece of metal stiff enough to put on blocks.

Give him 'steps" to climb if you want just keep varying the whole small table/board thing making it higher successive times and let both of them do it.   It's not just heights -- it's the whole foreign surface of the vets table that is slippery and noisy.  .  It will take you a while but you'll get there.  Don't EVER leave it with him being scared or refusing.  Go back to the last well-tolerated configuration and re-do that so you can leave him with a treat and an "awesome Dude!" and then next time figure out something to make a lesser step out of what he refused to do last time.  Insert something safer (like carpeting) onto the scarey surface and  and just make your steps between changes REALLY TINY and you'll desensitize that/

A lot of dogs are nervous on a table in the middle of the room. I shoved my grooming table into a corner, and now have two walls "enclosing it" and abra cadabra, the dogs are more still. You could start with something against a wall, and slide it out an inch at a time, as he becomes more comfortable.

 Tootsie doesn't like being on the vet table. Too high and she can't get traction, stainless steele. The vet would have a heck of a time examining her on the floor.o up she goes.

 Fear is dog´s normal self-protective response to potentially injurious stimulation, it´s natural and also dogs will freeze, attempt to escape or attack, depending on the fear-eliciting situation. To escape dogs to their fear ,It is so important to check dog´s heath (exercise, nutrition, activation etc.) first and sort out fear-eliciting situations and events .

Thanks