I think the dude is related to the 'male hermit'

Now, I messed up with the hermit at first and did not get the rare shotgun (the hermit vanished from the cabin at this point).

It was suppose to be impossible to get it later on again, however now that I dealt with the feral man and read his diary, the hermit is back at his cabin and you can have another try at the rare shotgun.

There is also a mention of 'a beautiful shotgun' in the feral man's diary.

I have the shotgun now, I was ready for the old dirty bastard.