You know in a situation like this before I made a sizable investment in an observatory I would pay for a survey myself. I think they cost about $300 which is small in relation to an observatory. That way you know for sure and have legal ground to stand on should he dispute it. After you get it surveyed you may find out his string and fence posts are on your land but at least the issue would be settled.
There are plenty of people that can run a property line. Ask around you friends and see if anybody is a Forester or knows surveying.
It seems from your description that it is a fairly simple line to run. That way you would KNOW if you have a problem.
At that point you can hire a licensed surveyor and get an official survey.
Do you have any survey posts on your property at all??
When you bought your house, you should have gotten a plat that shows distance and bearing angles. Finding a post between you and your neighbor makes it real simple,.
If you can find a post running a line is very simple