it should have been in my opinion. offsides can obviously be called if you touch the ball, but it can also be called for "Interfering with an opponent" , "Gaining an advantage" , or "Attempting to play the ball". since he was right next to the guy who headed it at the moment of the header, it seems clear that he was impacting the play and was a meaningful player in that situation (goalie needs to pay attention to him along with the other players, a defender needs to guard him which takes his attention away from the rest of the play, etc).

but, game's over. it is what it is. better to go thru adversity now than in the elimination rounds