Ah sorry i think I must have misunderstood what kind of pokemon game he was playing. It sounds like he's really into the card game?
If so, then it sounds like he's got a fantastic head for strategy on him! What a clever little lad.
I absolutely can tell you about what special interests mean and how it can affect the autistic person and those around them.
My special interest is gardening. Tomato gardening specifically. I am obsessed with growing different varieties of tomatoes, and many other plants as they all support eachother in a thriving ecosystem.
It's not something I can do passively, it's what I live and breathe.
When someone tries to talk to me about their interests it's not that I'm being rude or ignorant, the part of my brain responsible for reciprocity just doesn't engage, so I can seem a bit selfish, but it just takes me a long time to process what they're saying. That does mean though that when its their birthday or Christmas I usually want to buy them something related to something they've told me about, however because hobbies can change there's always a lot of anxiety as to whether they would still want the thing I've been thinking about all year for them and it's hard for me to think of anything else. I wouldn't ask them because that requires the emotional reciprocity and communication parts of my brain to switch on and work together. It's probably easy for them to assume though that in 5-10 years time I will still like tomatoes, gardening, and being outside. It's really intense interest.
When I am doing my special interest and somebody interrupts me, I become dysregulated, this is due to having my interest stopped but also because I have trouble switching between tasks. I might cry, I might get cross, or I might have the emotional bandwidth to mask however this is basically bottling up my emotions until somewhere later down the line they explode like a shaken up pop bottle.
When I'm not doing my special interest I'm in an hours deep Internet hyperfocus. It's monotropism. People could be talking to me, but my ears aren't hearing them. Their brains are polytropic, so they're probably watching tv, doing the dishes, talking about the weather, but my ears have switched off. It's a one track mind.
When people engage with me with my special interest, it's the most wonderful thing because I feel understood and the focus isn't on me, but is all on my special interest, but when people are just watching me, it's like eyes are boring holes into my head, because the interest isn't just on the activity, but on me too.
When people engage with me but their "rules" are different to my rules, this causes conflict because I have very rigid thinking. I got a lot of "just play nicely" "why can't you just get along" as a child, and it's because my brain needs rules to work, but the rules need to be adhered to by every one, and if they're not explicitly documented, and are rules you just assume, then of course my rules for growing tomatoes might be different to your rules. This would trigger my sense of justice and I would go into fight or flight mode.
Fight or flight mode is your lizard brain, and when you're using your lizard brain you can't be logical or reasonable or rational, but this is usually the time when adults use as a teaching moment, so it doesn't work, and the behaviour is repeated, because all that should be focused on is making sure the individual is regulated first and their lizard brain or defense instincts have calmed down.
Sometimes I have to choose not to start my special interest, because I know I will have to stop doing it in order to fulfil my roles, responsibilities and obligations. Not doing my special interest makes me feel restless and a bit depressed but not starting it to begin with feels more manageable than starting and having to stop and switch tasks, but both are incredibly dysregulating, compared to my NT peers who would probably just think "ah well, can't be helped" and move on with their day.
The emotional dysregulation, the lack of social awareness, and the intensity of the interests as well as the inability to switch tasks are the disabling parts, they're all very extreme and repetitive, and we often build them into our routines daily or weekly to ensure that we can participate in them, and then if our routines are broken or our schedules change we become incredibly dysgulated too. It's a vicious cycle.
To my family this feels like I am being controlling and manipulative, and I suppose I am but not with the intent to harm them, but it's a need not just a want, and it's a need I have to meet. It challenges what we can do spontaneously together, where we can be, how long we can be there. When I am down, they are down as they obviously care about me and don't want me to be sad, when I am happy they are happy but they're on edge to make sure routines and schedules are adhered to.
When I am dysregulated it can be mildly or can be extreme, and I don't know if you've seen the info graphic about the meltdown spiral but that's me. There's intense feelings, sometimes I feel out of control, then I feel like I wish I wasn't here, then I feel guilty and ultimately I'm in the pits, then when I come out of it I am exhausted beyond comprehension, my short term memory is skewed because I've just had a major traumatic event so my recollection of events might not be linear or I might be missing things out or my brain might fill the gaps with things that just didn't happen the way I imagined them. It can leave family members feeling unresolved. It can be very isolating.
I'm an adult with my own autistic child now, and I've got more tools under my belt to help me and I have had many bouts of therapy which hasn't always worked but every therapist I've had has taken a different approach and accumulatively all that experience has helped me, but your son is so young. He hasn't got a developed impulse centre in his brain. CAMHS are not operating as well as they should in many areas so he isn't going to get platinum levels of support from them.
You don't need to push your other child out to make space for your son, and I can appreciate more than any one how hard it is for family members of autistic individuals, as it seems they need to make more sacrifices compared to NT families, and when you've got other young children life can feel unfair to them.
I think both lads needs their own space and times to play their games, separately and together, always encourage play together, but make sure each of them has time to recharge away from eachother too, and they have things to share but also have special things for just themselves.
Get the visual chart somewhere communal, whenever it's quiet/solo time, point to it, make sure they know they have to go to their own spaces. Every one has to go do their own thing. When it's free play make sure they both know and they can go anywhere and do anything, and if your son tries to say his brother is watching him, you can remind him that this is free play time, the next time for solo play time is in 2 hours, why don't you come and talk to me about pokemon instead etc? Make sure you've got a logo for mum is busy dad is free, and dad is busy mum is free, so they know which parent they can go to and who is on duty. Make sure you also factor in time you need. This is the perfect opportunity for you to also see when your time to recharge is coming up and when it's time for dad to take over.