I think what's obvious to me now parenting first 2 compliant people pleasing girls (not saying they all are!!!) and then a little boy, who struggled to express his emotions and often lashes out in frustration (so far not in school, but at me) is that understanding how to behave and how to cope with understanding unspoken rules of behaviour does not come easily to some children.
In my case it's my son, but I've seen it in girls too - their strategies tend to be around needing to control others and performance crying, intimidating and bullying to get the desired result. For all of you saying "parent your child more effectively", it isn't always that simple. Yes, sometimes it's due to lax parenting. But not always. Some children need much more parental input and to be told explicitly how to cope and manage complex emotions.
It's not always easy when someone with more social nous and a bit of a mean streak sees your child as a target and takes great delight in winding yours up to the point that he/she explodes, then does the innocent hurt face. We've all come across that situation. My girls learnt how to manage the situation better instinctively, my son needed telling explicitly to stay away from that child, and how to (a) recognise when he was in the early stage of temper rising and (b) how to get himself out of the situation. He still frequently gets it wrong at 7.
So I helicopter parent him in social situations, whereas I've never had to with the girls. I only let him go on playdates with children whose parents are aware that he can struggle in certain situations, although, to be fair to him, he manages to hold it together in other people's houses usually. I make sure playdates here have a directed activity, frequent pitstops for snacks and a bit of time out. But it is exhausting and so we limit them.
OP, in your situation I wouldn't let him play out unattended for a bit. Watch him, watch his interaction with other kids and talk explicitly about what happened and the consequences. Be really really clear at what point he should have asked for help/walked away/changed tactic. The 5 point scale is really good - 1 is fine, 2 is a bit fractious, 3 is tempers rising, 4 is full on anger and 5 is physically lashing out. Teach him to recognise first 3, then 2.
Give him distraction techniques. So, for example with the boy stopping him on the trampoline, tell him to turn his back and bounce. It may be that this will wind the other boy up and he might try and escalate it - I try and teach my son a really silly joke that he can repeat to diffuse tension. Or a physical challenge to get away from the situation "I bet I can get to the back door without touching the floor". Or even something as simple as "wow, look at that plane, I wonder where it's going". The key is to try and distract in order to get away and to an adult safely before his temper rises to the point that he can't control it.
To other parents who are frustrated with "naughty" kids hurting theirs - please remember that all behaviour is communication and try to understand where these children are coming from, and your own child's part in it. There's always a reason - I'm not victim blaming by any stretch, but we will all come across people who behave badly throughout life and how we teach our children to manage them will, in turn, increase or diminish their power.
My sister was bullied horrifically at school - I hated her bully and wished her dead for the misery she caused my sister. Eventually, my sister stood up to her and that one tiny act of defiance switched the balance of power a little. It didn't completely stop it, but the bully could see my sister gain strength and she eventually looked for other targets.
As an adult I found out that the bully was being abused by her stepdad.
There's never a black and white - my child good your child bad in most situations.