I'm really only expereinced with cattle.
If he has 5 acres, you have 10 - make (2) 5 acre pastures out of yours, and the livestock can be rotated on each pasture for 15 days. It will take 30 days to get back to a pasture again. This gives the trees a chance to continue to grow, the grass to rejuvinate, etc. Perhaps your livestock can be added to the rotation and run on his property as well.....
Anything you work out with the other person will work for the fencing, but if you wanted me to build it I would sure want something in writing that I can use it long enough to pay off if I'm using my supplies - fencing isn't cheap, & you throw me off in one season....
Also, as the landowner, any livestock that escapes & is hit by a car will eventually involve you in a lawsuit. As landowner, you are generally held liable for the condition of the fence. Run any plans past your insurance company that you are indeed covered for this, esp if the other person is to be building & maintaining the fence. Sometimes that bothers insurance companies & you become a loophole with such an arangement.
Should all work out, just some things to get in order.
--->Paul