Adverse possession is the legal term for "squatters rights". Adverse possession means to acquire title to land through obvious occupancy of the land, while claiming ownership for the period of years set by the law of the state where the property exists. This law is codified in Indiana under Sections 32-21-7-1 et seq. and 34-11-1-2.
In Indiana, one has to occupy the property for 10 consecuitive years without the permission of the true owner. Also, Indiana law requires that the "squatter" pay taxes for the entire 10 year period.
If the person has not lived there for 10 years and paid taxes during that time, then he has no squatters rights and is actually a trespasser.
Technically, you could call the police and report that someone is trespassing on your property and he can be arrested.
If you don't want to pursue criminal charges, you can sue for civil trespassing, which asks the court to order the person off of the property, but no criminal charges or sentences are given. This may be the better route since you know he moved in with his grandmother.
The botXXXXX XXXXXne is that he has no right to be there, especially if he's not paying rent. Thus, you can have a court order him out immediately for trespassing.