Can people change?

I'm sorry but I need to get this out somewhere and I don't know where else to put it.

I apologise it's very long. Anyone who has time to read all this, advice, support, thoughts welcome.

Today is my partner's birthday and I don't think I have it in me to be positive and give him the best day.
I feel so alone and low and I don't have anyone I can talk to, because I'm worried they'll all say I need to leave my partner but after 8 years together we're engaged and I've been happily planning a wedding.
Thing is, if our arguments go on too long and he feels he's not being listened to and he gets upset by this he uses horrible language, screams at the top of his voice and when queried, claims he wasn't angry. We've been working on this for years. A lot of it was tied to drinking and I've seen where it's come from, his parents treat eachother and him the same. On boxing Day his father and him were calling eachother out into the street for a physical fight. Over the years the severity and frequency has reduced. I've learnt not to keep pushing a row until it gets to that, to wait and talk about things another time when he's calmer, he's learnt to go out for a walk before it gets to that. It's not perfect but it was heading in the right direction.

Saturday night we couldn't sleep because our neighbours were having the biggest party and he was talking about trying to join them, in a sort of 'might as well, we can't sleep anyway'. I thought he was joking at first and went 'oh yeah, go for it' then when he got up and started getting dressed I told him I didn't think it was actually a good idea, something about his manner made me think he agreed that it wasn't. I was very tired so when he left the room I just tried to relax for half hour/45 mins or so and thats when I went out to find my partner in the neighbours garden taking to the woman who's birthday it was, with her wanting to know why he was there and him saying he'd been invited to join by one of her guests.
He said later that he'd also been given a drink by this person, who had then left the garden when the birthday girl had come out, basically telling her it was her call whether my partner could stay.
My partner had hopped over the wall dressed but barefoot (he wanders around our garden barefoot but I know it looked strange to the neighbours because they commented). Been invited, been offered this drink by one person then apparently 8-10 people came out to the garden and started challenging him as to why he was there, backing him into a corner and not letting him leave when he said he wanted to. What followed was my partner trying to explain himself, but the more backed into a corner and misunderstood he felt the more he swore telling people to fuck off, the more confrontational he became. he came got onto the wall to come back into our garden and guys from to the party were jeering, my partner said he was going to kill them just before he landed in our garden and once in our garden the guys from the party came up to the wall with one jeering ones challenging him to a fight.
so my partner then tried to get back over the wall and I tried to hold him back but not before he'd exchanged blows across the wall with a guy who was looking for a fight. Eventually got my partner back inside but he was furious ready to take it back out onto the street, with a plank of wood that we have for his allotment. I managed to keep him inside.
As he started to calm down he swung between angry and sobbing, saying he had just been trying to do the right thing. That he was invited over and handed a drink then they just suddenly turned on him and backed him into a corner and just set him up to look like the dickhead.

Someone in the neighbours garden said that they called the police, but if they did, we didn't see them. I'm now scared we'll still hear from them.
My partner can't acknowledge that he did anything at all wrong to contribute to the situation.
I'm scared his 'fight the world' 'stand up for what's right even if you're standing alone' mentality will land him in prison.
I love him but he doesn't think he needs any help. I don't want this kind of drama in my future. I don't want him screaming at me anymore, however rarely. I certainly don't want his triggered behaviour to call into question the safety of our home and peace with neighbors.
I don't know what it is that means he acts like this, is it a personality disorder, is it a spectrum condition, is it depression, is he hiding his drinking. He maintains he hadn't drunk anything before going into their garden.
Are people where he grew up so very different? Have his parents damaged him so much? They are both alcoholics and the language they use is awful and the toxicity of their way of interaction are identical to these worst ways in my partner.
I love him and the majority of the time he is my rock, I don't want a future without him, I want to raise a kid with him but when something like this happens as it has over the years, but not with the neighbours, I can't see a future. When he screams at me, i can't see that future. I've seen him improve and change especially in the last year but now this has happened, I don't know what to think or what to do. 😥