This is not an uncommon reaction but to be honest -- and given your description of Bentley's relative isolation from other dogs -- you will probably want to immediately do something to address this or he will likely become worse -- and potentially more dangerous to other dogs. This can be a very serious problem indeed because if he attacks and harms another dog in most places, the dog wardens can demand the dog be put down. But even if things do not reach that serious a level, it is not much fun to have a dog that constantly mist be managed around other dogs. Even by this description, he is now a dog that may need to be muzzled on walks if he cannot interact happily with new dogs on walks? (the Baskerville muzzle is the best and allows dogs to easily drink and breathe but the owner needs to train the dog into being comfortable wearing one).
It is vitally important for all dogs to regularly meet other dogs (meaning NOT just the same few dogs in the house or friends' dogs, but new strange dogs, every week or every day). When dogs do not get properly socialised, and then remain socialised regularly (daily, ideally, on walks) to new dogs, they can become gradually more and more withdrawn from being any part of a dog world, and worryingly and even dangerously aggressive (which is a sign of anxiety due to lack of socialisation -- this is not very pleasant for the dog or the owner). Because they either don't meet enough dogs when growing up and/or remain well-socialised to a new dogs (never only just the household dogs or the same friends' dogs) dogs can fail to know how to interact successfully and sometimes even to know how to read normal canine meet-and greet signals. They can become aggressively protective of their owners, home or items, and for the dog, life becomes ever more narrow and limited when they do not have the chance to interact happily with other dogs. The broader worry too is that they then do not have good self control towards humans either as they don't have or slowly lose the built-in good level of, say, natural bite inhibition they should have, which is what in turn makes them know good bite inhibition around humans. Dr Ian Dunbar and all good trainers are very adament on these important interactions and need for at least weekly interaction with other dogs over a dog's lifetime.
If Bentley or anyone's dog, has been limited in who he sees, and you have had this alarming reaction to a visiting dog, I'd want to work with a good trainer to see if this can be addressed as it will make him and you a lot happier and enable you to enjoy walks without worries and have friends visit with their own dogs (all part of the pleasures of having a dog
). It may be as simple as starting by taking a group obedience class where he can gradually lower this level of reactivity. I'd look on the CCPDT website for a qualified trainer doing classes in your area, as a good starting point, or asa second choice, the APDT website for same.
I would not leave the situation as it is. If there'd been a serious fight this would have been distressing at best and potentially disastrous. Someone who is not a family friend may not be as understanding, and things could end up in a very difficult spot.
I do know what this is like as my Lily (who came from the pound) is reactive to new dogs (though not actually aggressive) -- have had her evaluated by qualified trainer friends because of this and also took her to numerous classes, starting with sitting on the edges. She gradually had her reactivity levels drop way down and she is regularly in home boarding where she is fine with other dogs though can be rude initially... but well socialised dogs just ignore her. But she is a pain on walks and I have to manage her -- as owners can be offended and also, a poorly socialised dog might interpret her as aggressive and go for her. It can help enormously to get involved with a good trainer and the dog will be so much happier, better able to interact normally.
It is so much easier for any dog owner to socialise puppies very well -- at least 100 people in their first 12 weeks as Ian Dunbar advises, and other dogs as soon as they can do daily walks, and vaxed older dogs from the start, in a safe place. Then to keep a dog well socialised through classes and walks and visits. Trying to fix a problem due to poor socialisation after the fact, as I know from having Lily, is far, far harder. Sometimes it is not possible and then the responsible owner must manage the problem dog safely around other dogs as a result, which is no fun.