New cat mom, and I am so worried

We’ve adopted two 16 weeks old kittens today. They are foster brothers. One of them is trusting and exploring, but the other one is mostly hiding under the couch. He is a very sensitive boy who has experienced a lot of trauma from early age. He was very close to his foster (human) mom. Is it normal that they are this timid and what can I do to help him? I am a worried (almost) first time cat mom and my heart breaks for him 🩶💜

Photo is him being up and about a few minutes, after I caught him and held him tight and close (after advise from his foster mom). He is back under the couch now.

Also photo of his more confident brother