He already has a body type at 18 that is less concerning than nhl players like Turris who have finished growing. He will likely have less issues with getting outmuscled than guys about the same weight who are taller, for example Krieji, Turris, Marchand, connor Brown, Haggelin, Simmonds, Palmieri, Rielly Smith, Justin Williams, Namestnikov, RNH, Barzal, and the list goes on. The concern about height is the ability to fill out and put enough muscle on the frame, but he's already done that.
He's also already played against men, albeit as a 16 year old in the tier 2 swiss league.
Size concerns are overblown, he already has the strength, with his low center of gravity he will be fine in board battles and fighting off checks. The drawback is you can't project as much growth because of getting bigger but there are lots of guys bigger guys that are drafted mostly filled out, this is no different.