If he is getting good accuracy it may not be tumbling at all. Could be an odd tear in the target as mentioned above. If he is getting all odd shaped holes and poor accuracy, they may be tumbling.

First, I'd back off to 100 grains of powder. If he wants to extend his range, he could go to loose powder charges up to say 120 gr. For normal ranges like within 100 yds, I'd go with 100 gr.

If that doesn't cure it, I'd switch bullets. Of course, this all assumes the barrel is in good shape.