Did you ever tell him that he is expected to chip in? Maybe he genuinely did not know because he didn't had the experience of money being tight. If money is no concern, having people try to pay you back is the socially awkward thing. I once had a gf that didn't understood that on movie night, she was supposed to bring her own food potluck style but who also casually invited the entire friends round to a weekend trip to Disneyland because "wouldn't that be cool?"