My ds (10) is in the fourth grade; he has an IEP.
I think that it would be a reasonable request of his next teacher. That being said...
I don't know about your school board but my DS is just finishing gr. 6 and I'm sure that it's been at least a couple of years since he brought spelling words home. I think that his class (and his gr. 5 class last year) are working on spelling rules and homonyms, ect. I think that teaching the rules of spelling and encouraging reading is a better way to learn spelling that memorizing a list of words every week.
Both my DS and I have ADHD but we can both memorize things very easily. Trouble is that a month later we don't remember what we've memorized. He and I are both avid readers and so spelling is reinforced that way. My having learned common spelling rules (and of course the exceptions LOL) has helped me greatly in having mostly correct spelling. DS is still learning but I'm sure that it will help him later.
Perfectly reasonable, I'd say, and you'll know if there are 5th grade spelling words before that anyhow.
Lest you have the same experience as us & 5th grade homework is much longer than 4th grade, you do know you can get short-term "boosters" of most meds to cover the afternoon homework slot? It's a lifesaver for us since my kid tends to wear off around 3-3:30 & now in 6th grade it can be 2-3 hrs of homework some nights, and when it's long, it's always writing, where he has to be on meds.