, +R refers to positive reinforcement. It's just another method of training that adds a stimuli (such as a treat, pat, scratch, etc.) as a reward. The opposite is -R, negative reinforcement, which removes stimuli as a reward. An example is using pressure and release. Hope this makes sense! :)
The best thing to do is to go with other horses, even with one horse if possible.
What "+R sessions" means ? Thank you :)
It could be either fear or barn sourness, but in either case, a couple +R sessions should nip it in the butt.
To me this is a reaction to fear. Will's advices are nice, it is easier if you start by hand walking, you can better show him the places and he will appreciate the fact he can see you.
Some horses frighten themselves outside, we cannot always understand why. As you say he stops and makes a u-turn, I think he is afraid. So, try to go for a walk by hand, don't stay on his back. It can reassure him if you stay near his head or shoulder as you can really see you. You can also bring some friends and eventually their horses. Do this 2 or 3 times per week, it can takes some time, it will depend of your horse's personality.