However, I am not surprised he didn't do well at the test, as he finds it difficult to decode words he cannot adscribe meaning to.
Hah! Now you've got me thoroughly confused! This implied, for me, that he either was unable to decode and blend a word that he didn't recognise, or, that he tried to make unfamiliar words into familiar ones.
Yet you say:
He can decode and blend, and does decode a blend very complicated, new words, even things he is not remotely familiar with.
Which is, of course, absolutely as it should be 
So I'd revise my comments to:
If it's the first scenario - worry
If the second - he sounds fine and was probably just a little 'off' on the day, or maybe the task wasn't clearly explained to him.
Or perhaps he expects words presented to him 'in school' to make sense (because maybe the teacher emphasises 'meaning' a lot) while words on the outside he knows will frequently be unfamiliar
Do you know what his score actually was? Was it close to the threshold or way off it?
SnowieBear,
I can see where you are coming from, but I'm not at all sure that we should be expecting 6 y olds to be able to judge if a word is 'nonsense' or not. I'd like them to just rise to the decoding and blending 'challenge' and find out afterwards what the word does (or doesn't) mean.