Was the paper from the new NFER/GL pack? My DS2 also had a dip in scores straight after returning from holiday and like you I panicked a bit, but have since heard that these newer NFER papers are felt to be harder than the old ones, so a reduction in scores isn't particularly surprising.
I would use it as an opportunity to discuss with him the fact that he will most likely find the real papers difficult and not to panic - just keep going etc. He shouldn't expect to score 99% on the day! He doesn't need to.
I've decided not to do any more papers with my DS2 now. He's probably done about 12, and if that's not enough, then so be it. Like Bemily, I will let him "play" on freerice a few times a week and probably won't be able to resist discussing vocab from time to time
, but certainly nothing too taxing.
Chin up - almost there!