He’s young and has barely any training. Considering that, if he doesn’t hold back he’s almost as strong as a normal Viltrumite. So soon he’ll be really strong

Besides him being young and lacking combat training, he hasn’t really been working out. Viltrumites can get stronger and faster if they push themselves. If he starts a workout regimen and gets some more lessons in combat (something The Immortal could help him with) he’d stand a better chance.