I mentioned it previously, but I'm more concerned with Djoos' explosiveness than his strength. I'm a believer that undersized defensemen can succeed in the NHL (and people have cited numerous examples), but in order to do so you need to be able to beat players to pucks and have the hockey IQ to navigate out of difficult situations. I'm not sure if Djoos is that explosive skater to achieve that. Maybe his anticipation can overcome some of that, but he needs to be able to beat opposition to pucks and out maneuver them at the blue lines. Even if he packs on some weight, he's never going to be big enough that it's going to be an impressive portion of his game.
Plus I think Troy Mann said something to the effect of "he can train all he wants, but at some point you have to admit the weight just isn't going to stick." He's been around professional training staffs, including the Capitals organization, for several years now. Some people just aren't that big.
He just needs to spend a season playing baseball.