Feed him more!
See how he manages with small crickets.
Dust cal Ca and Vit D 2-3 times a week!

Puzzling !
A 1 year old Whites should be much larger.
Your suspicions may be right!

What other species of 'green' , small..... tree frogs are available in the area you reside?

Can you post additional photos so we can help you accurately ID the frog.

Is there a reason why he is kept in this QT- type setting?

You have heard and witnessed calling? If so, have witnessed an enlarged 'vocal sac' during the calling?