This year, I subbed in two different leagues. In league one (1) I bowled 30+ games. League was very competitive and a joy to bowl. I could have gotten into brackets and side pots, but have cut back on that in the last year. Don't expect to get anything back and don't mind.
In league two (2), I bowled 60+ games and almost qualify for year end awards (might actually since I am an extra member of the team). Even if I do, I will have the secretary remove my name from any awards list since I didn't pay any league fees. Once again, don't expect anything and don't mind.
As a sub, I realize I do not pay any money and thus am not legally entitled to any monies. If your sub only bowled one night, I am sure he wouldn't expect any part of the prize fund, so it makes sense it would be the same if he bowled more than one night. If he is still insistent on getting a share of the prize fund, perhaps mention the fact you paid for lineage you didn't bowl. Perhaps he would be willing to pay for the lineage that he bowled.
If any sub expects any prize fund money, it MUST be agreed upon before bowling the first night. That is the only way it is fair to the person paying the fees. If nothing was agreed upon, then there is nothing to be paid.