Keep it short and from the heart - Say, "I know you are hurting and I am so sorry that you are having to deal with this terrible loss." Tell him that you know words are pretty much useless in soothing his hurt but that you want him to know that you do care about him and value his friendship. If you knew the Dad or had any type relationship with him, you might add that you will also miss him and appreciated the impact he had on your life.
