@DitheringMusicParent I think parents have to accept a few things around very young musicians and theory: (a) with a few exceptions most of them are averse to the idea. (b) the ABRSM G5 theory is out of date, dry and boring. (c) with young ones I have found theory is best tackled in a short block(s) if you need to heavy lift them through the exam. (d) if they pass it, it is unlikely to have any great benefit to any future musical plans they have (again I am sure with some exceptions) because they have forgotten what they have crammed by about 3 weeks later. (e) if they are too little for it you have to decide as a parent whether to go through the misery of making them do it anyway, or whether you shrug, accept that, revisit it when they are a bit older and recognise that despite the fever that can develop around talented little ones, it actually doesn't matter.
Two of mine did their G5 theory at 10 because of imminent G6 exams. I definitely would not have wanted to tackle it with them any younger than that. With both of them they were mature enough by then that we talked about it and discussed how much it really mattered. They both chose to give it a go because they had their own ambitions around their practical exams. I think that it was important that they had made the decision themselves and were therefore willing to put the effort in. The elder dropped in, did a short online course, didn't find it too hard and passed about a week later (she is very good at picking up concepts in subjects like maths). The younger one processes things completely differently, had to do two courses and work much harder but did get there.
I found having to do the exams at that age counter productive because it put them off theory. Funnily enough my eldest developed the greatest aversion despite finding it relatively straightforward. My younger one, because she had done more of it, did get some more secure understanding and sense of pride in grasping bits of it.
Overall it is a shame because theory, when learned alongside the playing they enjoy, is actually interesting and valuable. My eldest is now 14 and the grasp she has of theory has come about in more recent years through discussing the concepts in an applied way with her teacher as she learns repertoire.
My youngest is 9. We will come upon the G5 theory question in the next year. When we do we will have the same conversation about whether it is something he wants to do. At the moment he would not want to do it and I would not want to make him. I probably could, with bribery and coercion, but I suspect we would both be miserable. The sudden cognitive leaps they take at that age (9-10, 10-11, 11-12 etc) are huge. In a year's time we might be in a different place but we might not. But he hands down loves his playing and that is great.
My reading of your message is that you don't actually think your son is ready to learn theory at that level and, if that is the case, that's fine. He can still love his playing and develop as a musician. It won't hold him back in the future. He just won't have a piece of paper at 9 and will either have to hold off ABRSM exams or carry on with Trinity. No music school in the future would give a hoot about that. My younger daughter was at a JD when we went through the G5 process. If I had said to them that actually I thought she wasn't ready, they really wouldn't have cared. They aren't fussed about exam grades. They are much more bothered about the damage that can come about from pushing children when it is counter productive and making happy future musicians unhappy child musicians. And you may well find that your son, at 10 or 11, is in a completely different place.
I think music leads to choices about what sort of parent you want to be. There are other JD parents who, I am sure, would take a different approach and have done what it takes and got their child through G5 theory at 7 to have them at G8 by 8. I just think that's a bit nuts and sounds deeply un-fun for everyone. For me music is about enjoyment, the immense valuable lessons about teamwork, resilience and life it teaches and the sheer pleasure of being part of something so fabulous.
I do think that it is important not to get too swept up in the future with little ones and music, however talented. They are their own people. If it is a passion he will find his way with the support you are giving him. And he might grow up to be a great doctor or save the world doing something else. None of mine want to be musicians and if they said they did, I would want them do go into that tough career with their eyes open, because they love it and because it is a positive choice made by them as adults, not made for them as children by the adults around them.