Why make him a sub - that’s the point. How on earth does it help him or us?
This has happened many times, I even see other sides doing the same with their first gamers.
Personally I don't think its a bad idea so I don't have a problem with it. Giving the young blokes the sub vest lets them experience the game before they get thrown into it. When they do get a run they are matched up on a bloke who has already run 7kms or so, giving the young bloke an advantage. They can go hard knowing there is only a half or even a quarter left to play.
We talk about the importance of development but don't want to see it or have to put up with it! We talk about green shoots but expect them to magically appear on the field as fully grown crops ready to yield fruit ...