We may need bigs, but that doesn't make everyone we recruit a center. This kid isn't a 5, and if we told him that it's no wonder he's still looking.
There's no reason to think anyone told him he's a 5 - only Boneyard fantasies. It's quite likely they see him as a stretch 4 a la Deandre Daniels. Defend PFs and provide offense from anywhere on the court (interior, perimeter, transition).
Offensively this is a world of difference. Playing with your back to the basket on offense is a totally different skill set than playing facing the basket. Mamoudou has spent nearly all his time on offense facing the basket, and you can see in his practice videos he's facing the basket.
Defense is much more a position-independent capability. If he wants to be a good player, he has to learn how to defend all kinds of players. Switching is rampant so perimeter guys end up defending big men close to the basket. As a stretch 4, you're not really asking him to do anything a 3 wouldn't do, just differences in the proportions of activities.
That's why players who get fixated on a position ("I'm a 3", "I'm a PG") are really doing themselves a dis-service. They should aim to be versatile. Basketball players, as KO says, not position players.