Should I report this guy as a hacker?

[Toxcatl]

i understand how it's possible to do this when a player is very young. Not sure I understand how it's possible when he's this age? You received players from the academy with very low skills and hide them on the bench power training only certain skills. How would you do this with an older player?

You train a player, get a promotion (he loses 20% in every skill), then you train him again, get a promotion, train again... Just a few seasons, and he has 1% in some skills.
Actually, it is harder to get young players with skills on 1%, there is a special trick (upselling), but you need to have several teams with 2-3 levels difference. Low-level team buys a 3* player, trains him, sells to another team (the same manager) a few levels higher, so the player again becomes 3*, but his grey skills now lower. Then training and selling again to another team a few levels higher (the owner is the same manager). Repeat a few times, and you have your young mutant. Expensive, takes a lot of micro-control and detailed preparation, but 100% possible.