High Frequency Word 'Scores' and age? advice pls?

[firstchoice]

Have tested my 9.7 yr old on the first 100 High Frequency Words
(then the next 200 too).

He scored:
78/100 for the 1st lot (beginning: the, that, not, look...)
then 58/100 for the next lot (beginning: water, bear, find....)
then 48/100 for the last lot (beginning: let's, fun, any, better...)

some mistakes were phonetic, eg, 'becos' but lots were not phonetic, and there were a lot of missing vowels and letter transpositions too.

School are just now reluctantly 'going over' some vowels with him.
But he says: 'the "rules" don't make sense, mum'.

Is the above worrying, and how best to help him, please?
thank you.

[CecilyP]

You might get a better response if you post on Primary Education.

[firstchoice]

thanks, CecilyP.
Have done so.