Re: Men that try to push sports on kids... what do you do?
OP, I'm curious as to the age of your son. Is he not at the age your FIL can take him to a game. I'm a Philadelphia Eagles and Flyers fan, although I don't watch them religiously even if they are having a winning season, My son has one Eagles onesie and I do not own any sports memorabilia at all. While you may not enjoy sports I assume that if your son was to play a sport you would enjoy going to your son's games and if he was talented would enjoy watching him be successful.
I'm not sure what you are considering too much merchandise. You said enough to fill a box, boxes are many sizes. You said you guessed it to be around $250, that could be four items. I'll assume each child is getting about ten items a year. Too much is definitely a problem, but I'm not into Disney memorabilia and if someone were to get me two items a year to me that would be too much.
I imagine your son being 15 and playing football, in that event he might look at the plethora of sports gifts from a different perspective than you who does not have any interest in sports at all. If your son was going to these games with your FIL and your DH, this would also change their perspectives on how much meaning the gifts had.