Underweight? How to Put Weight on a Beardie?

This is a follow up to a post I made a few days ago. I’m concerned that my Beardie (5 years old) is underweight compared to pictures I see on this subreddit. I just weighed him tonight, and he ticks in at 620g.

Is he underweight? I don’t have a length as I can’t find my stupid measuring tape. Here are a few reference pics, also taken tonight.