Cat aggressive only when leaving the house

Hi, I am concerned about my cat's very aggressive behaviour when he leaves the flat. Bit of background, he is a 3 year old rescue cat, involved in bad car accident, had a broken pelvis so probably has considerable trauma in his past.
When he wants to go out, it is impossible to walk near to him or past him without him hissing and scratching and making a strange miqowing noise that alos involves coming towards me with his front paws sort of waving around. Usually I can avoid him but not always and it is unnerving to say the least. I have tried hissing back but that riles him even more - oh dear. I looked at other aggression threads. It could be redirected aggression as there are a lot of other cats around although he is not aggressive to them when he is outside. He does turn into a different cat. I haven't been telling him off as I assuemd it was some instinctive reaction but are there things I can do? The same kind of things mentioned on general aggression threads? Any ideas, Thanks, Tara

Why don't you just keep him indoors now - going out seems to be a hardship for him (and who could blame him!) and why risk further injury?

Oops, didn't make it clear. I've had him for 18 months now and he loves to go out! He definitely becomes more crazed if he can't go out. The problem with the aggression going out got worse when we moved (a year ago) as before we could just leave door/window open for him but there are too many cats here to do that. I would like to know how to stop him doing it or do I just resign myself to him always being like this? It's more of a problem when he have people with kids around and he is somewhat erratic at times.

Is he neutered? If not getting that done will help a lot.

Yes, he is neutered. I wondered if he came from a home where he used to be kicked when he went out?