This might be a little controversial, but I think NAH.

As the father of a son who is having issues with his weight, he was trying to show support for his journey by considering the struggles he might have if the food at the wedding was high calorie. John wouldn't enjoy himself if he's surrounded by food he can't have (no it's not John's wedding and it's not really important whether he is enjoying himself over Robert, but as a parent, dad was considering both children). It's a huge internal battle, and it can be really difficult. Dad is trying to be supportive of this. Dad's request honestly seems to have innocent intentions. He was just trying to look out for his kids (no, I don't think this makes it seem like dad is prioritizing one kid over the other as long as dad took no as an answer, and let it go).

On the other hand, Robert is NTA too because it is his wedding and he has more to consider here than just one guest. He has every right to not go with his father's idea. Robert wants to enjoy himself, and while he can't please everyone, he wants the majority of his guests to enjoy themselves too.

Obviously this isn't written with the context of how he mentioned it. I think if he wasn't pushy, and it was just a suggestion, he let it go after Robert said no, and he didn't ask John to talk to him and John did that on his own accord, then NAH. Now, if he kept pushing and forced John to talk to him, then I would consider say YTA, and take no for an asnwer.

I really wish we had more context on how the message was delivered and wether they kept pushing.