Thank you for your question. First, a gentle reminder, “an” is used before a vowel. You would use “a” not “an” before disappointment.
The answer to your question would depend on the speaker. If the father was the speaker (or writer in an email or letter, for example), he would be saying that he tried to teach his son. The father believes that fathers are supposed to do so but his son turned out to be a disappointment. If the son were the speaker (or writer), the reverse would be true, it would be his father who failed him.
I think you meant to say “failed you.” Not “failed with you.”
Failed with you would have an entirely different meaning. It would mean that both parties failed together.