The real question is, how did the attacker have inish to play Sons of Thunder... 8/6 > 1/1
The other remark is, since King Zedekiah makes Dungeon CBN, then Sons of Thunder couldn't interrupt it even if John had init.
However, in case you are curious: Say you block James and John with Beast from the Earth (10/10). You play Dungeon to capture James. John plays Sons of Thunder. Since the interrupt pauses the effect of the enhancement, James has not been captured yet, and X=2 when SoT completes.
If you block with King Zed (or some other guy that makes Dungeon CBN) and let's say he is somehow >7/8, then you play Dungeon to capture James. John now has initiative to play SoT, but he can't interrupt Dungeon, so X=1.