If it's the HFW list from L&S it can probably be grouped to make it easier. And he can probably manage about 1/3 of them already.
About 1/3 of them are simple VC, CVC words - a, an, at, if, in, it, on, can, dad, had, get, big, him, not, got, up, mum, but. If you are in the north you can include 'put' with this set too.
Then I'd go with the words with ll, ff, ss, zz, ck at the end - off, back, will
Words with the spelling 's' for the sound /z/ - is, has, his,
Words with adjacent consonants - went, from, just help, and,
Words with ch, sh, th, ng, wh - that, this, them, then, with, when
Words where the /o/ sound is spelt with an 'a' - what, was,
Most of that I'd expect most children to be able to do by the end of reception, but may be worth recapping especially is, as, his, was, want, and possibly when. Get him to sound them out when he's writing them down. It will help secure it in his memory.
Then most of the rest can be grouped by sound, looking at the different spellings each sound can make e.g.
/ai/ - they, make, made, day, came
/ee/ - see, he, me, she, we, be, people,
/igh/ - I, time, my, like, I'm,
TBH giving out words by where they exist on the 300 HFW lists is a bloody stupid idea if that's what they're doing. There are things like 'hot' and 'sun' that are going to appear on the third list and can be spelt by most YrR children in their 1st term and things that are best taught together e.g. was, what, want/as, is, has, his/have, live, give will appear split across lists.
And almost all of it would fit into their phase 5 phonics teaching so could be part of the lists they are probably sending home anyway. Grrrr. Why make things harder than they need to be.