I have 3 indoor male cats. They are perfectly behaved and really happy. They have never shown signs of aggression and they run away from the front door when it's opened.
I do however do things to ensure they are happy and enriched. They are also all neutered.
Feliway Optimum Plugins are really good for getting through a stressful time (I don't find the other versions effective).
I also really recommend Feliway Cystease Capsules. You just open them up and sprinkle into their wet food. It has a calming effect and helps with any urinary issues.
Scatching posts, cat trees and toys are important. And it's good to get new ones every so often as it is exciting for them.
Unneutered male cats can be perfectly behaved for a certain amount of time and then bad behaviour suddenly starts. I always, always recommend getting them neutered ASAP, but even getting it done now will help once those hormones leaves his body.
Did the vet check his eyes? One of my boys has bad eyesight and can growl if he can't see who we are. He isn't aggressive, but he will growl until we properly come into view or we speak.
Also some cats do suffer with hunger aggression. I had a rescue that would bite me viciously when he got hungry (was perfect otherwise). This was almost 100% solved by switching his diet.
I now don't feed dry food to any of my cats. I had a great conversation with a specialist vet at the pet hospital in Edinburgh and he said that a 100% wet diet is the best. Most of the urinary and kidney issues he sees would be solved by ditching the dry.
High meat content, grain free is the best.
I get Wild Freedom, Feringa and Macs from Zooplus. It's a great site and the tins are really handy.
I've noticed improvements in all of their happiness and they stay full for much longer too.
Good luck OP.
I would also get some high value treats like Thrive Freeze dried Chicken and start re-bonding with him slowly.