Theseus' face relaxes only marginally, but it's clear by the worry in his eyes and the fact he's not letting Newt go, that he's going to remain by his side for a while longer. If Newt was fine, he wouldn't just suddenly collapse onto the ground. He may play it off as no big deal, but Theseus isn't going to allow himself to be fooled by a few smiles and 'assurances' that his brother's really fine.

In fact, his grip on Newt's back only tightens when he realizes that Newt is trying to brush his well-being off in order to get Theseus to stop worrying.

"If you wanted me to stop worrying, you shouldn't have collapsed in the first place." Maybe he just let it slip a little that Theseus hasn't stopped worrying ever since their encounter with the moles, but he's not ashamed to let Newt know that he's concerned about his health.

"You can tell me if your injury is bothering you, Newt." He swallows audibly, the memory of Newt protecting him and his blood spraying all over the ground-- the infection taking root, still fresh in his mind. But Theseus forcefully shoves that memory back and he's glad that his hand is gripped into a fist behind Newt's back, just so that the other won't notice his hand shaking from the memory.

"...Or is it something else?"