Has anything changed in the house?
New furniture, rearrangement of items, etc?
If he's scared, then he's only going to feel safe in his home which is his safe place. If both budgies are now at a normal weight, it would be best that you start feeding them in the cage so the one that is afraid (name would be helpful) will be able to eat when you aren't around.
Molting definitely will contribute to budgies being more lethargic.
Keep an eye on him to ensure he is eating, drinking and pooping normally. Is he sitting all fluffed up when he's quiet?
When you are particularly concerned about one of your budgie's behaviors, it is a good time to take it in to your Avian Vet for a well-birdie checkup to ensure there is nothing amiss.
Please keep us posted.