Whatever the two of you agreed upon verbally is not possible to enforce. This is because agreements as to property must be in writing, per 740 ILCS 80.
This means that he's had his cabin on your property for some time.
But just because he has, does not mean that:
1) He can claim more land, or
2) He can claim an easement
(around the cabin, for use).
The only way that he can claim an easement is if he goes to court under a special QUIET TITLE action, and proves an easement by prescription
"An easement is created by such use of land for the period of prescription as would be privileged as if an easement existed, provided the use is (1) adverse, and (2) for the period of prescription, continuous and uninterrupted." Restatement of Property, section 457, quoted by Roller v. Logan Landfill, Inc., 307 NE 2d 424 - Ill: Appellate Court, 4th Dist. 1974
NOTE that it states ADVERSE, meaning without permission by the actual owner of the land (i.e. you). It does not matter that you thought he owned the land. In fact, the mistake is in your favor, because had he been doing this without permission, then the prescription easement is more likely to stick.
Therefore, he is arguably trespassing/encroaching on the land, and may be mandated by judgment to move back to what is HIS land.
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