I actually feel like his only real verbal response to her speaks volumes (to us, anyway). She confessed, he remained silent. When she pressed him for a response, he thanked her for her feelings for him. It shows how conflicted he is. On the one hand, telling her the truth about how he feels about her would be cruel, given the odds stacked against them. On the other, flat out responding that he doesn't reciprocate those feelings would be lying (and also too much of the noble idiocy we'd much rather avoid). Instead, he chose to go with an answer that's not really an answer, but is also an answer that leans towards the latter - that he does not reciprocate her feelings.
His response also perhaps reflects that he was hopeful - which is why despite his answer that's not much of an answer to her, he slowly allowed her to get closer to him, until finally when it became clearer to him that the odds that were stacked against them to begin with was stacking even higher. We saw him slowly start to put his walls back up, retreating and putting more distance between them, with him not being forthcoming despite his promise to her.
It's cruel, in a sense, giving her hope and then pulling the rug from beneath her feet, going so far as to tender his resignation and "escape" to Cleveland without any warning, but then again, he may view it as the kinder of two options. Given the choice, she'd likely put everything on hold to go to Cleveland with him, but I don't think he wants her to do that for him. He likely thinks that she's had enough sadness in her life already - with what happened to her father, and now having to think about ending the life prolonging treatment. After her father's accident, she wanted to escape to Madagascar. What would happen if she watches his condition deteriorate and subsequently possibly passing away before her eyes?
Hindsight is 20/20. Had he known beforehand that all of this would happen, he might have decided to respond differently. However at that point, he was a man who has CIPA and knows that he's lucky to have lived thus far. They were drawn to each other by a common situation where they decided to make similar choices. No one had ever come close to understanding him, and I don't think he even wanted her to know about his condition to begin with, despite his budding feelings for her. Her accidental discovery led them to have a shared secret, and even then, he had all these walls up, not wanting her to get too close. A moment of selfishness was all it took for him to let her in.
I don't think he necessarily regrets having let her in despite everything, but I think the last thing he wants is to see her cry for him.