I shot my sons 835 this weekend and it was shooting around 15" high. It is a Mossberg 835 with a 20" barrel along with a Indian Creek .675 choke. The only thing that changed was the shot shell. Used the londbeard by Winchester 3" #5. Had been Xtended Range. I shot an old round and they were high as well. I used this gun for many years and took plenty of birds. My ? Is this I adjusted the sights as far as they could go and it helped some about 8". If I move the rear sight forward will that help any at all? Don't know what has happened to the pattering. Any advice??
I don't have a clue what could be wrong with your gun all of a sudden after all these years, have you tried another choke? might be something with it. If you don't find what is wrong with it soon there is still time to put a scope or red dot on it and get it patterned before turkey season starts, if that is your sons only gun. From what I have seen and heard that Winchester 3" #5 should give you a good pattern. Wish I could be of some help, Good Luck