Hello there,
We have a nearly 3 year old Bernese dog - Barney - who all of a sudden has decided he does not want to go for a walk!  (He seems perfectly happy and healthy in all other aspects).  We get to the wood (and we have tried many different entrances) and let him out of the car and he just sits down and will not budge.  Sometimes, he decides to sit down half way round his walk - which is extremely trying especially when you are late for an appointment.  No amount of coaxing or bribing will make him move.  When my partner had not arrived home the other day after two hours I went to find him and he and Barney had been stuck in the same spot for almost 2 hours!  Any tips anyone??? 

Have you tried walking him in a different place to the woods ?  maybe something scared him.  Try walking him somewhere else to see how he is.

I would take him to the vet and have his health checked.  Excersise intolerance can be a symptom of many things, a heart condition, overweight etc, none of which may be noticed in the home at rest.

Our previous newfie used to just sit on a walk sometimes for a few minutes before carrying on.  He turned out to have a heart condition, so as the previous poster said a trip to the vet might be good idea.

Is he active at any other time? It could be he has pain in his hips/back/joints? As he is a large breed they can be prone to these things i believe. Best to get him checked out by a vet to make sure.