Oh, he addresses the letter to you, dad. 'My Dear Sir I'm truly grateful for all you've done for me. You've been so kind.'
10 seconds sound clip from the Five Fingers of Death movie soundboard.
You can hear this line at 00:41:19 in the DVD version of the movie.
Quote context
[...]
- You know I'm no match for you. Let's forget the idea.
- Come on!
- Dad, there's a letter from Chih-Hao!
- Come on, read it to me! You lads carry on, huh?
- I should have stayed at home to help you like a dutiful son.
- I was sad to say goodbye, and I'm thinking of you, and Ying Ying every moment of the day.
- As Sun Hsin-Pei's pupil, I had to work hard, and it's thanks to your training that I can keep up.
- Right now, I'm concentrating on training, for the big tournament.
- I don't know what I'd do without Ying Ying's encouraging letters.
[...]