Are they tiny signs it's coming, or is it out of the blue? Just wondering (ha! that word again) whether if you treat him as his emotional age, which is perhaps much younger, and stick like glue - is it then possible to see it coming or is it out of nowhere? If he can't foresee or link consequences, I'd have to supervise play as much as if the child were a toddler I think. What are things like at school, is it happening there?
It sounds like an impulse control issue as much as an anger issue - worth a referral back to the LAC paediatrician to see if they will assess, do you think? (I can almost feel your heart sinking at those words if you've already been through so much with your elder child, sorry...)
I read the thing about clapping too and I liked it - it's dependent upon being right there in the moment, isn't it, and spotting the opportunity to deflect? Not always possible. I guess I would probably keep play with others quite brief, so interrupt often to sort of take the emotional temperature, and do something like very long walks/bike rides, and very short playground time, if that makes sense? So still fresh air and exercise, but mainly alongside you.
Nottheduchess generally time out is avoided for children who are fostered/adopted/have experienced trauma. The neurological impact of their early experiences (including in utero) can lead to a heightened shame response and fear of abandonment. By using time out, you can trigger feelings of shame, rejection and fear. This is highly likely to escalate whatever the behaviour was that you were wanting to stop in the first place. Instead, you can use time in, where the same "complete stop" is achieved, but the adult stays very close, sometimes holding the child on their lap or sometimes alongside, and they have a think together. This is also because often our children don't have the early experiences of their caregiver soothing them, so they don't learn to self-soothe or later to regulate their emotions. We need to go back to that very early baby stage and soothe and regulate alongside them.