My son’s school (a charter) just asked to  move his annual IEP meeting from the end of the year to later this month because he moved up to a more advanced ELA class and is “an Armstrong student”

I will ask the school but would like to hear from experts here, too.

I'm not familiar with this term. Is it the name of a special program or an award?

The school said An Armstrong student is just a category of students who have a disability such as autism, and we have to determine ESY eligibility by February 28th. 
 

last year they determined he didn’t need extended school year and this year he’s doing even better academically.

i feel there’s an ulterior motive 

I'm also not familiar with the term "Armstrong student" and am not real clear what advice you're asking for, but I can tell two things.  One, there is no set date by which ESY has to be determined.  If they claim it's "school district policy," ask for a copy of the policy.  It is good practice to discuss this early, but February doesn't even allow you to look at spring break data.  Which leads me to number two.  The determination of ESY has to do with the regression the student has over breaks (winter, spring, summer) - not the progress he makes during the school year.

I heard it called Armstrong Group.  It's something specific to PA.  ESY has to be decided by 2/28 if your child falls into certain high need IEP categories.  You can download the ESY booklet here:  https://www.pattan.net/Publications/Extended-School-Year-Services-in-Pennsylvania  Info on Armstrong Group is on pg 8 & 11.  It refers to a lawsuit:  Armstrong v Kline.