How did you manage to get an IEP without a diagnosis? For something else?
We're in Canada... not sure if that makes a difference. Schools here will provide an IEP without a diagnosis. Funding is a different story. My son doesn't qualify for additional funding because in spite of his recent diagnoses, he doesn't test "low" enough (he doesn't test as being learning disabled). He gets time with the LA teacher and shared time with a TA based on the IEP, and time with the district Speech Language Pathologist because of his language disorder diagnosis. Not sure how that happens without extra funding... my guess is there is room for a certain number of kids within the existing $$ and they've included him because of the psycho-ed assessment.
The TA and LA time, as well as the IEP, came as a result of a team meeting per the teacher's request (because he was/is wiggly, quirky, impulsive, and has trouble paying attention and poor social skills... also intensely curious and prone to fixation). They/we originally thought SPD or ASD. The resulting team meeting and IEP were done prior to his psycho-ed assessment and diagnoses.
I think, frankly, the entire process was initiated by the school to get funding (for ASD). This has since been ruled out, but the wheels were already in motion for an IEP.
Here's an important point... he's NOT getting straight A's like your son, mainly because of his language processing disorder and the fact that's he's in French immersion. That could be the key difference. He's a math monster

Interestingly, if I had put him in Montessori and they accelerated his math, he may not have gotten the IEP and resulting support for his ADHD and language processing disorder.
Not sure if I've done the right thing, to be honest. He's got a truckload of support, his language skills are improving wonderfully (he's still behind in French but very strong in English), but his math, his big talent, is not being accelerated. His grade one teacher suggested I switch him to Montessori so that they could accelerate his math, and I was so torn.
If I had put him in Montessori, he'd be who knows how far ahead in math, but his English language skills might be weaker (i.e. less linguistic stimulation to compensate for his language disorder?) and he'd have zero French. He likely would not have been given an IEP because I think the main reason for that was because of his struggles with French. He would have been the math star with weak language skills, but now he's the kid with two languages who is at rock solid grade level in math (and science).
Ah, I don't know. Non-accelerated gift but a second language? Only one language but an accelerated gift? I just don't know.
Anyway (sorry for the novel, lol), the more I dissect it, the more I think that his struggles with French, combined with the school's quest for ASD funding, could be why he got the IEP before his diagnoses.