Hm, that's a hard call to make. My own personal experiences on the matter may be a little different from my fellow contemporary G1-era fans; I was the only one in my family who was a Transformers fan, and my parents didn't want to sit through the movie with me, just so I could see it. So while I pretty much "had" to wait another half-year before I'd see it at a friend's birthday party, I ended up seeing much of Season 3 already. But during summer of '86, there were already the trailers and TV spots in the movie that kind of telegraphed Prime's death before the movie hit theatres, plus there was the Marvel comic issue that was coming out, too. So at that time, even without having seen the movie during its initial run, the temperature of the water pretty much already told me, "Prime's gonna die."
Your son is probably in a different boat compared to back then - he may or may not have seen or heard those "advance clues" that I picked up from the marketing at the time; also, I was 10 when the movie was being promoted, so I may have been better prepared to handle death in a work of fiction. If he had trouble handling the Blitzwing fight scene in the
Bumblebee movie, I suspect he might not be ready for Optimus' death scene in the '86 movie just yet, or even the Autobot shuttle massacre before then. On the other hand, mayge he'll cheer Starscream's death scene (unless he likes Starscream, which would just twist the knife). It may also depend on if he's seen death scenes in other animated features, like the death of Bambi's mother, for a random example.
I'm thinking, maybe just replay the S1 and S2 episodes a few more times, but space them out a bit to buy yourselves some time, before you feel he may be ready to see the '86 movie. Maybe to extend that "stall", perhaps encourage some creative activities for him to do after watching a few episodes (e.g. maybe drawing some artwork). I can't predict when he may be ready for the movie, since I don't know how long he can build a "tolerance" or reach the proper level of "desensitization" for his age and/or psychological well-being, even for an animated feature. Let's just hope he won't get too bored watching the same eps over and over again until you feel the time is right. And maybe, to build up the preparation toward letting him see the movie, perhaps indicate that things will change a LOT, and some of the key players might not survive to the end.
Not a foolproof plan, but hey, it's all I can think of off-hand. Good luck!
