Cat attacking neighbourhood cats

[Bullycat]

We rescued a 5 year old neutered male cat about 4 months ago. We don't know anything about his background so are unsure what type of environment he lived in before.
Ever since we started letting him out he has been territorial, but in the last couple of weeks it is getting out of hand. He is fighting & chasing away all the cats in the neighbourhood who have been here for years. Yesterday he entered a neighbour's house through the window. This neighbour has 5 cats but he got in & attacked the female cat. She is really badly scratched & we feel awful. We have kept our cat inside since but he is not happy being an indoor cat & is constantly trying to get out.
Does anyone have any ideas on what we can do to stop him being so territorial & aggressive to other cats? He can't be an indoor cat as it will drive us & him crazy.

You can try zylkene capsules sprinkled into his food, it’s very palatable.

Cat proofing the garden is effective if you get the right system, agile high energy breeds need different systems. You can diy or have it fitted. Does the neighbour use an open window as a makeshift cat flap? Microchip cat flaps keep them safe in their own homes.

Usually cats sort out territories themselves eventually, I’d keep him in overnight to be safe.

Build a catio or cat-proof your garden.

We have cat proofed an area of the garden. (Still have to be careful opening gate to main part of garden). We used Purrfect Fencing uk. On occasion he tries to climb but their arch system at top preventing him getting over. We set fence top high...
As a firm they are very helpful. Not cheap but not the most expensive either....