All of our three boys have played flag football. We just got back from our
9 y/o's game.
What you've described is pretty common, and if that is all there is to it, it's not bullying. Even though it is flag football, I notice that after our practices, the boys keep playing and sometimes they get a little rough. They push, pull, grab, and tackle. They're boys. And not just that they are boys, they are boys who like football. It kind of comes with the territory.
As long as your son and the other boys are not singling out X and making him the recipient of all of the physicality, I do not see this in any way as bullying.
You've done everything you can do. You've talked to your son, his teacher, and the other boy's father. Both the teacher and the coach have denied seeing any evidence of bullying.
I do think you are wise to keep talking to your son and to caution him about being less "hands on" when it comes to X, because now your son is on X's radar, and even an innocent physical interaction may be cause for this dad to think your son is picking on his son. I'd also keep talking to your son to ensure there is nothing else going on (like subtle things, or excluding X, or what kids sometimes think of as "friendly" name-calling. There was a post about this the other day).
It's a shame that some people take a serious issue like bullying and then try to
frame every unpleasant interaction their kid has under this umbrella.
Best to you and your family.
J. F.