Hi Justnormal:
We had a very similar problem with DD1 in year 1 and our solution was to make up two separate sentences. One sentence with 'oi' words and another with 'oy' words.
So with your list:
Disappointed
annoying
voice
checkpoints
choices
joyfullness
avoiding
marmalade - not sure why this is with 'oi' words - so probably would just learn this separately.
would - also not an 'oi'/ 'oy' word - so again would learn this separately.
So first off you and your DC should select out all 'oi' words.
Disappointed
voice
checkpoints
choices
avoiding
Avoiding using your voice at checkpoints when making choices can leave the officer disappointed.
It's a nonsense - but if they learn the sentence can learn the 'oi' words. Now in our case we have 5 'oi' words and 5 'oy' words so 2 sentences made more sense.
In fact you only have 2 oy words: annoying and joyfullness.
Tom's sister's joyfullness at seeing Tom fall over was really annoying.
So in fact if your DS makes up a similar style of sentence with the 2 'oy' words - he basically knows all the other words are 'oi'. If there not in the 'oy' sentence they're 'oi'. Then learn the two words without oi/oy sounds: would and marmalade and your home free.
It's learning to the test - but we try to focus on the meaning (so we read through the list and make sure that we understand all the meanings - especially if a word has more than one meaning - I haven't gone overboard on noun/ verb/ adjective/ etc... - but I am starting to sneak it in with DD1 who's in Y4). We also work on imagination/ functional grammar (practice making up the sentences and insisting they make some sort of logical sense) and writing (practice writing these sentences). By putting structure into a list we've found our DDs can then sort out words into groups - can learn these groups - and then can do well on the test.
At first, DD1 (now Y4) seemed to just learn these for the test and then instantly forget them, but 2 years on we're actually starting to see accurate spellings and a good sense of vocabulary. So spending this time does incrementally seem to have benefits.
HTH