Blocked anal glands was my first thought too. Does he scoot along the floor on his behind? This is usually a sure sign of trouble with anal glands.
As to his agrophobia - sounds like the poor dog was dumped somewhere far from home by his previous owner!
Have you tried walking the 100 yards he is comfortable with, then going quite literally 1 or 2 yards more. Then make a great fuss of him for being a good, brave boy, and telling him, "Now we shall go home. Find the way home, boy!" and let him go home at his own speed, whether this means running as fast as you can, or him sniffing at every weed, lamp-post etc. to find his way.
Then a big fuss of him when he finds home. After a week of 102 yard walks try 105.
If I am right, that he was dumped somewhere and could not find his home again, he should become more comfortable when he realises that he can find his way back, and if not ´, you can and will lead him home.
Or is he all right walking, but scared of the car? If that is the case, definately a "dumped" dog!
Is he more confident if one of the other dogs is with you?
I can see the difficulty of getting him to the vet, when he is scared to leave his home - how long have you had him?
My father had an Alsatian that had been dumped - it took two years of patiently putting more food in that dog's bowl when he had eaten a little of his dinner to get him to realise that he was welcome to his food, and although he loved my parents, Rex , if he got out of the garden alone, did not dare come withing arm, or foot's lenght of them ever. Daddy got the dog to come, by parking his car with the back door open and walking round the next street corner. The car was fixed in Rex' mind as the marvel that had transported him from the police pound to his beloved home, so safe to enter.