It's a difficult question, and an unexpected burden that Toby's unintentionally laid at his feet. He can't say what he "should" to a question like that, because it isn't the truth and wouldn't feel right in these moments of excruciating honesty and sincerity, and especially after admitting that he knows he can't overpower Toby under the worst circumstances.

"I love you." He takes a long moment to consider his words, and then offers entirely earnestly: "What I know is that you decided to die, and then came back and lived again. Part of that was because of me, but not all of it was." That, at least, is honest and something he can say without the danger of it feeling like a platitude. He takes a breath, his hand shifting to grip at the one fretfully wrapped around his own. "I think you're stronger and more beautiful than you can see."