He posted a letter to his dad, Johnnie, then later that day found out that Carla needed a transplant. Next morning he intercepted the postman and got the letter back - suicide note, farewell letter?
He's often distant during family gatherings and does seem depressed. Several times his dad has asked him if he's okay. Carla was disappointed by his reaction when she handed over the factory to him, he was flat and despondent.
Aiden was crying in the hospital bed after the transplant, he's been obviously down at other times, too. Poor chap.