You're not alone. I've seen my Dad naked while changing before and after swimming, but I've never seen his cock erect ... nor have I wanted to. I really don't care how my size compares to his. And I do know that my tool is considerably larger than average regardless of the size of his endowment. He did fit the size issue into his birds-&-the-bees speech he gave me because he knew that it would come up (well, so to speak!). While he spoke about penis size, he turned red and was obviously embarassed ... we both were. He stuttered his way through the condom lecture, and told me that if I ever had doubts when it came to buying appropriately sized condoms, then talk to him. However, I'm sure he was thinking, "Dios mio, I hope he never asks me to go condom shopping with him!"
Some issues, including cock size, can be awkward topics for father-son talks, but after it was over I think Dad was glad that he broached the subject (sort of) with me. I think we more glad that he got through it!