There are two issues: one is your son preparing to successfully accelerate, and two is getting the school to agree to acceleration.
I’ll talk a little about the second issue first. As background reading, search for “Mathematics Framework and Acceleration to Higher Mathematics” (it will be at the California Department of Education website) or for a more detailed discussion, “Appendix D: Course Placement and Sequences of the Mathematics Framework” (also at the CDE website). As you’ll read, the California official stance on accelerating in math as of the adoption of integrated math and the Common Core is inherently rather negative, though there is some somewhat more accepting wording in Appendix D, wording which you may find useful when talking with your school. Regrettably, it doesn’t sound like your school district is offering any of the suggested acceleration pathways. [And this is why the CDE talk of “equity” regarding the CC/IM math pathways is so disingenuous: knowledgable parents in high-performing school districts ensure that their districts offer and their kids are placed in compacted, accelerated classes and tracks before or during middle school. Whereas kids in other school districts can end up “locked out” of straightforward options for accelerating in math, and then at something of a disadvantage for college admissions and college success. Grrr.]
You mention that your son tests well. That’s good, because as several posters have already mentioned, your son should plan on being able to demonstrate his readiness for IM2 by taking and doing well on the 9th grade end-of-year IM1 test. In the absence of taking a class labelled “IM1”, it is extremely likely that showing he can do well on the end-of-year IM1 test will be necessary to convince the school to allow him to go directly into IM2.
As far as learning the IM1 material goes: an Algebra 1 class isn’t going to cover exactly the same material. I suggest also taking a look at Aleks Math, which offers an online IM1 course. In theory, you could learn all of IM1 online with Aleks; in reality, I’d have grave doubts that most kids would do well with just Aleks. However, it could be an efficient way to find and fill in any “gaps” between what an Algebra 1 class covers and what IM1 covers. But take a look now (I think Aleks offers a couple hour free trial – maybe get through the diagnostic test) and check where your son is at now. Taking an actual algebra 1 class over the summer is probably a good idea (unless the diagnostic test shows your son is really, really strong at algebra already!) – he needs to have a really solid algebra introduction to keep progressing in math. But if he remains motivated enough (and not burnt out on summer math by the algebra class), he’ll still need to follow-up on the algebra 1 class to be ready for the non-algebra, alternate material that’ll show up on an IM1 test.
Aleks is well-suited for kids who intuit math concepts, don’t like/need much drill, are self-directed and curious, and just need to fill in gaps. The Aleks theory is that it just naturally expands one’s math understanding without really needing lectures/detailed explanations, using an artificial intelligence engine to keep presenting kids with the “next step” of math; it sort of works, but when a kid does hit something that really isn’t making sense to them, the terse Aleks explanation probably isn’t going to be much help. (So if you were trying to rely on Aleks alone to learn IM1 material, I’d expect you’d need additional resources – maybe also hire a tutor to supplement with direct instruction when appropriate.)
Finally, it is true that your kid doesn’t necessarily need to get to Calculus in high school to get into engineering in college – but lacking it is a disadvantage compared to the other kids who’ll be applying who have taken Calculus. If trying to accelerate at the beginning of high school doesn’t happen, one more way to get to Calculus in high school is to take a pre-calculus at a community college or even online (if sufficiently motivated and responsible) the summer between junior and senior year. Pre-calculus is a course that, thirty years ago, was an optional course: extra-strong students sometimes skipped it to go straight to Calculus. I don’t recommend skipping it entirely – lack of adequate pre-calculus preparation is probably the biggest single problem for students in Calculus. But it’s a class where the community college version of the class should be more than adequate preparation (possibly better) than the high school year-long version. And I’ve known several kids who successfully studied pre-calculus online over the summer to get ready for Calculus senior year.