Letting a taker take even once just teaches them they can take and move on to the next victim. People need to prove themselves as trustworthy before giving them anything. Offer them something relatively small like buying lunch. If they don’t hesitate to take it then they’re takers. Someone who won’t refuse at least twice has no qualms about taking for nothing. But I just don’t associate with people enough to allow them the chance to even try.
There’s a good chance he has something going on causing him to have a distorted perception if reality. I deal with a lot of people like this who have paranoid delusions and often seem manic in the manor demonstrated in this post. What exactly have they done to treat it so far?