Our 7 year old recue dog won't go toilet when out for a walk?

We have rescued a Lhasa Apso, he can be walked up to 3 miles and not go to the toilet but if he's in the house and left for even 2 minutes he will go. I can understand some of it is separation anxiety as he was with his last owner from a pup, then put into a kennels but how can we get him going to the toilet when he's walking