But I would not pin everything on the fact that he is not neutered..
We had the same issues and had our dog neutered and did nothing for his aggression. If fact he is usually the most agressive in spring & fall, when other dogs are in heat. I find agression is due to the dogs original litter standing and the pack order of your dog in your family.
We used the squirt bottle and it worked really well. He rarely growls or goes after anyone anymore. If he does he gets squirted and we show no fear towards him. We also do it without anger and put in in his crate for a long time out. It has helped a lot, but he still tests us from time to time. I doubt he will ever really change, we can change though and we control his behavior not leaving anything to set him off or the possibility to have an incident. You can never let up, EVERYONE has to be on the same page about who's boss and consiquences must be inforced at ANY sign of aggression.
I would never leave either of our dogs in the back seat with a stranger. They are a lot like people, they don't want some fool stranger staring at them, fussing with them or invading their space. They care little about what you or anyone else thinks.
Most people think they own a regular dog, but my experience is they are not, they are more like owning a tame badger. Would you trust anyone alone with your tame badger?
I don't, why chance it?
I find our dogs do great with some people and dogs and others it's just trouble, so I just gave up and when people are over they stay in their crates or on walks try I avoid people & other dogs, or make them sit while they walk by.
In my opinion he's not going to change that much, but you can, and don't put him in those situations. You just can't put an adult basenji in the back of a car (uncrated) and expect everyone can do what they want, like eating, petting or touching him. Not everyone knows what you should not do to set a basenji off (staring in his eyes, touching his tail, surprising him while he's asleep etc) It's not going to happen in my opinion. Again basenji's are more wild dog / human than pet. If some stranger came up & touched your tail, what would you do? I would be mad as hell and punch him in the nose! Basenji's are the same. They don't enjoy strangers unless they feed them treats and even then it's not like they can do what they want.
Realize you own this instead of a pet dog