I know you are concerned about your dad's illness, but he will be all right.
Everything will be fine so don't worry and take care.
心配は英語で:'worry', 'trouble', 'anxiety',といった意味になります。相手の心配を軽くするために、心配だけどもきっと乗り切れるという表現を使ったほうがいいと思います。
例:'I know it's tough for you to see you dad going through this tough time, but he will
be all right'.
'Stay positive and you will see everything will be fine'.