Mathis has been on the books of Metz since 2015 and made his professional debut last season. The 19-year-old forward featured in two Ligue 1 matches during the 2016-17 campaign, playing a total of 20 minutes, according to WhoScored.
Everton recently completed the signing of another young forward. The Toffees secures the services of 20-year-old Henry Onyekuru from Eupen for a transfer fee reported by BBC Sport to be worth £7 million and immediately loaned him out to Anderlecht.
The Toffees have been quite busy in the summer transfer window so far. Ronald Koeman’s side have signed Davy Klaassen, Nathangelo Markelo and Jordan Pickford, as well as Onyekuru.
The Merseyside outfit will be aiming to break into the top six of the Premier League next season and perhaps even challenge for the Champions League places.