Personally T I'm not sure a response is called for or needed as he has said he will phone you later in the week. I would just trust him on that and wait for him to take the initiative.
Or if you wanted to say something I would just keep it short and say something like
Dear B
Nice to hear from you. Thank you for the birthday wishes! I'm sorry to hear that you are sick though, I hope you have an instantaneous 100% recovery!!
No problem about the call, whenever is good for you. Safe travels.
T
And
HAPPYBIRTHDAY
Sorry I missed it, I hope you had a wonderful time.
