Upvote:1
You are supposed to fill out the form truthfully and accurately to the best of your knowledge. If your father is traveling with you on the same ticket purchased in one transaction, there is no IF's, BUT', WHAT's about it. You definitely have to mention it.
If your father bought his own ticket and just happens to be taking the same flight with you, you could argue that you are not traveling together however the chances are that using a reasonable man standard, it very likely will be construed to be deception on your part to indicate he is not traveling with you.
Basically, fill out the form truthfully and don't attempt to misrepresent.