“He has to adjust to a proper physical pre-season, which is hard for him so we help him and do the right things for his development. “
I don't think that means "bulk up", really. He's a Dutch kid coming from a Dutch league, so a lot of his conditioning would have been in the form of various small sided games, instead of the intense non-ball fitness drills that Klopp uses, so I think that's what he was referring to - a "proper
physical pre-season", as opposed to getting him into the weight room to increase muscle mass.
Core strength development is, though, a good shout, as he seems quite lanky to me, and will definitely need to improve that to cope with the physicality of the English league and cups.