The first thing I would do is get him checked over by a vet to make sure there is nothing physically wrong with him.
There is a technique called T Touch or Tellington Touch which can be used to calm animals.
It sounds like your dog is stressed over something, maybe something has unnerved him that is not obvious to you.
I would try and find a good trainer/behaviourist who can work through this with you. The more you force him into situations the worse you will make him. It may need you to totally back off from walking him for a while if possible, then reintroduce him slowly to things like people and cars etc. stopping the minute it starts to stress him out.
For more in depth advice immediately a good place is here:
http://www.ttlntl.co.uk/forum
They have a number of behaviourists/trainers who will give you really good advice - it is totally free to join and they are really friendly!