He’s a child.

What will you have in 500 years?

482 more years of lifting fat weights.

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

He holds back. That’s his problem. Viltrumites are killers. He has to learn to be one to beat them.

he’s a baby in comparison to other viltrumites who’ve been alive for hundreds if not thousands of years

Literally every punch thrown by a Viltrumite is intended to kill immediately. They never hold back even one iota. They believe in irrational force, which is supposed to be a deterrent. But it’s not, as we see in the series. There’s always resistance. And it’s always eventually successful.

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.