I agree with the "saying what is on your mind" part for sure. The way you say it though is the key. Try to be casual about it rather than coming right out and saying "Dad, you really need to practice more since you have not been able to hit a deer with that bow." Ask him to go out and practice with you on occasion because "You have been having trouble getting your groups together like you once did." That would put him in the role as educator instead of his being the student. Even if he does not help you much in this regard at least it will let him approach the situation from an angle that he is comfortable with.

Second, you mentioned the Whitetail Extreme. I may be wrong but isn't that a Cabelas or Bass Pro special from PSE? If it is then did you have it set up correctly when it was purchased?